Apparatus for forming a blank for finish forging for a forged crankshaft for a three-cylinder engine and method for manufacturing a forged crankshaft for a three-cylinder engine using the same

ABSTRACT

In a forming apparatus, movable journal dies and stationary journal dies retain rough journal portions of a preform blank therebetween, and reference crank pin die and movable crank pin dies contact rough crank pin portions thereof, and in this state, the movable journal dies and the movable crank pin dies are moved axially toward the reference crank pin die and the reference crank pin die and the movable crank pin dies are moved in a direction perpendicular to an axial direction. With this, weighted rough arm portions are axially compressed to reduce their thickness to that of weighted arms of a forged crankshaft, and the rough crank pin portions are pressed in the direction perpendicular to the axial direction to increase an amount of eccentricity to that of the crank pins of the forged crankshaft.

TECHNICAL FIELD

The present invention relates to techniques for manufacturing, by hotforging, a crankshaft (hereinafter also referred to as a “forgedcrankshaft”) for a three-cylinder engine. In particular, the presentinvention relates to an apparatus for forming, in the process ofmanufacturing a forged crankshaft, a blank for finish forging to besubjected to finish forging by which a final shape of the forgedcrankshaft is obtained, and a method for manufacturing a forgedcrankshaft for a three-cylinder engine including preforming steps usingsuch forming apparatus.

BACKGROUND ART

In engines of passenger cars, motorcycles, agricultural machines, andthe like, a crankshaft is required for taking out power by convertingreciprocating motion of pistons to rotary motion. Generally, there aretwo types of crankshafts: those that are manufactured by forging andthose that are manufactured by casting, and the former forgedcrankshafts superior in terms of strength and stiffness are more widelyused. In recent years, in order to improve fuel economy performance andmeet emission regulations, downsizing of engine displacement becomespopular, and a three-cylinder engine is attracting wide attention.

In general, forged crankshafts for three-cylinder engines aremanufactured by using, as a starting material, a billet having acircular or square cross section and having a constant cross-sectionalarea along the entire length, and subjecting the billet to the steps ofpreforming, die forging, trimming and coining in order. The preformingstep includes roll forming and bending, and the die forging stepincludes block forging and finish forging.

FIG. 1 is a schematic diagram illustrating a typical conventionalprocess for manufacturing a forged crankshaft for a three-cylinderengine. A crankshaft 1 illustrated in FIG. 1 is to be mounted in athree-cylinder engine. It is a three-cylinder four-counterweightcrankshaft that includes: four journals J1 to J4; three crank pins P1 toP3; a front part Fr; a flange Fl; and six crank arms (hereinafterreferred to as “arms” to be simple) A1 to A6 that alternatively connectthe journals J1 to J4 and the crank pins P1 to P3 to each other, whereinamong the six arms A1 to A6, first and second arms A1 and A2, and fifthand sixth arms A5 and A6 respectively connecting to first and thirdcrank pins P1 and P3 at opposite ends, have balance weights. The thirdand fourth arms A3 and A4 connecting with the second crank pin P2 in thecenter have no balance weight, therefore having oval shapes.Hereinafter, when the journals J1 to 14, the crank pins P1 to P3, andthe arms A1 to A6 are each collectively referred to, a referencecharacter “J” is used for the journals, a reference character “P” forthe crank pins, and a reference character “A” for the arms. An armhaving a balance weight is also referred to as a weighted arm whendistinguished from an arm having no balance weight. On the other hand,an arm having no balance weight is also referred to as a non-weightedarm or an oval arm.

According to the manufacturing method shown in FIG. 1, the forgedcrankshaft 1 is manufactured in the following manner. Firstly, a billet2 shown in FIG. 1(a), which has been previously cut to a predeterminedlength, is heated by a furnace and then is subjected to roll forming. Inthe roll forming step, the billet 2 is rolled and reduced in crosssection by grooved rolls, for example, to distribute its volume in thelongitudinal direction, whereby a rolled blank 103, which is anintermediate material, is formed (see FIG. 1(b)). In the bending step,the rolled blank 103 obtained by the roll forming is partially pressedin a press in a direction perpendicular to the longitudinal direction todistribute its volume, whereby a bent blank 104, which is a secondaryintermediate material, is formed (see FIG. 1(c)).

Then, in the block forging step, the bent blank 104 obtained by bendingis press forged with a pair of upper and lower dies, whereby a forgedblank 105 having a general shape of a crankshaft (forged final product)is formed (see FIG. 1(d)). Then, in the finish forging step, the blockforged blank 105 obtained by the block forging is further processed bypress forging the block forged blank 105 with a pair of upper and lowerdies, whereby a forged blank 106 having a shape in agreement with theshape of the crankshaft is formed (see FIG. 1(e)). In the block forgingand the finish forging, excess material flows out as a flash frombetween the parting surfaces of the dies that oppose each other. Thus,the block forged blank 105 and the finish forged blank 106 have largeflashes 105 a and 106 a, respectively, around the formed shape of thecrankshaft.

In the trimming step, the finish forged blank 106 with the flash 106 a,obtained by the finish forging, is held by dies from above and below andthe flash 106 a is trimmed by a cutting die. In this manner, the forgedcrankshaft 1 is obtained as shown in FIG. 1(f). In the coining step,principal parts of the forged crankshaft 1, from which the flash hasbeen removed, e.g., shaft parts such as the journals J, the crank pinsP, the front part Fr, and the flange Fl, and in some cases the arms A,are slightly pressed with dies from above and below and formed into adesired size and shape. Finally, the forged crankshaft 1 ismanufactured.

It should be noted that, when adjustment of a placement angle of thecrank pins is necessary, a step of twisting is added after the trimmingstep.

With such a manufacturing method, however, it is inevitable thatmaterial utilization decreases since large amounts of unnecessary flash,which is not a part of the end product, are generated. Thus, in themanufacturing of a forged crankshaft, it has been so far an importantobject to inhibit the generation of flash to the extent possible andachieve improvement of material utilization. Examples of conventionaltechniques that address this object are as follows.

For example, Japanese Patent Application Publication No. 2008-155275(Patent Literature 1) and Japanese Patent Application Publication No.2011-161496 (Patent Literature 2) disclosure techniques formanufacturing a crankshaft, by which journals and crank pins are shapedand crank arms are roughly shaped. In the technique of Patent Literature1, a stepped round bar having reduced diameter regions at portions to beformed into journals and crank pins of a crankshaft is a round bar usedas a blank. Then, a pair of portions to be formed into journals, betweenwhich a portion to be formed into a crank pin is disposed, are held withdies. In this state, opposing dies are axially moved toward each otherto compressively deform a round bar blank. Concurrently with impartingthis deformation, punches are pressed against the portion to be formedinto a crank pin in a direction perpendicular to the axial direction,whereby the portion to be formed into a crank pin is placed into aneccentric position. This operation is repeated in succession for allcrank throws.

In the technique of Patent Literature 2, a simple round bar is used as ablank. One end of the two ends of the round bar is held with astationary die and the other end thereof with a movable die, andportions to be formed into journals are held with journal dies andportions to be formed into crank pins with crank pin dies. In thisstate, the movable die, the journal dies, and the crank pin dies areaxially moved toward the stationary die to compressively deform theround bar blank. Concurrently with imparting this deformation, the crankpin dies are moved in an eccentric direction perpendicular to the axialdirection to place the portion to be formed into the crank pin into aneccentric position.

With both the techniques disclosed in Patent Literatures 1 and 2, noflash will be generated, and therefore a significant improvement inmaterial utilization can be expected.

CITATION LIST Patent Literature

-   -   Patent Literature 1: Japanese Patent Application Publication No.        2008-155275    -   Patent Literature 2: Japanese Patent Application Publication No.        2011-161496

SUMMARY OF INVENTION Technical Problem

As described above, according to the techniques disclosed in PatentLiteratures 1 and 2, a round bar blank is directly processed into acrankshaft shape. However, forged crankshafts are required to have highstrength and high stiffness, thus blanks for the forged crankshaft arenot easily deformable. As such, crankshafts that would be practicallymanufacturable are inevitably limited to the ones having arms of largethickness and crank pins with a small amount of eccentricity, andtherefore having a relatively gentle crankshaft shape. Moreover, all thecrank arms are limited to a simple shape without a balance weight, thatis, an oval arm.

In addition, according to the techniques disclosed in Patent Literatures1 and 2, the shape of arms is formed by free expansion of a round barblank in a direction perpendicular to the axial direction in conjunctionwith its axial compressive deformation and by tensile deformation of theround bar blank in conjunction with the movement of portions to beformed into crank pins in an eccentric direction. Because of this, thecontour shape of the arms tends to be unstable, and thus dimensionalaccuracy cannot be ensured.

The present invention has been made in view of the above-mentionedproblems. Accordingly, in order to manufacture forged crankshafts forthree-cylinder engines with high material utilization and also with highdimensional accuracy regardless of their shapes, it is an object of thepresent invention to provide an apparatus for use in forming a blank forfinish forging to be subjected to finish forging on the premise that, inthe process of manufacturing the forged crankshaft, finish forging forforming its final shape is performed. Further, it is another object ofthe present invention to provide a method for manufacturing forgedcrankshafts for three-cylinder engines with high material utilizationand also with high dimensional accuracy regardless of their shapes.

Solution to Problem

A forming apparatus according to embodiments of the present invention isan apparatus for forming, from a preform blank, in the process ofmanufacturing the forged crankshaft for a three-cylinder engine, theblank for finish forging to be subjected to finish forging by which afinal shape of the forged crankshaft is formed.

In the forged crankshaft, third and fourth crank arms connecting with asecond crank pin in a center have no balance weights, and remainingcrank arms have balance weights.

The preform blank includes:

-   -   rough journal portions having an axial length equal to an axial        length of journals of the forged crankshaft;    -   rough crank pin portions having an axial length equal to an        axial length of crank pins of the forged crankshaft;    -   third and fourth rough crank arm portions corresponding to the        third and fourth crank arms of the forged crankshaft, having an        axial thickness equal to an axial thickness of such crank arms;        and    -   weighted rough crank arm portions corresponding to weighted        crank arms having the balance weights of the forged crankshaft,        having an axial thickness greater than an axial thickness of        such crank arms.

The apparatus for forming a blank for finish forging a forged crankshaftfor a three-cylinder engine according to the present embodiment furtherhas the following structures (1) or (2).

(1) The rough crank pin portions in the preform blank have a smalleramount of eccentricity in the direction perpendicular to the axialdirection than an amount of eccentricity of the crank pins of the forgedcrankshaft.

The forming apparatus includes a reference crank pin die, movable crankpin dies, and stationary and movable journal dies, described below.

The reference crank pin die is disposed at a location of a second roughcrank pin portion, configured to be brought into contact with the secondrough crank pin portion, and configured to move in the directionperpendicular to the axial direction, but be constrained from moving inthe axial direction, while being in contact with side surfaces of athird and a fourth rough crank arm portions through which the roughcrank arm portions connect with the second rough crank pin portion.

The movable crank pin dies are disposed at locations of thecorresponding first and third rough crank pin portions at opposite ends,configured to be brought into contact with the first and third roughcrank pin portions, and configured to move axially toward the referencecrank pin die and in the direction perpendicular to the axial direction,while being in contact with side surfaces of the rough crank armportions through which the rough crank arm portions connect with thefirst and third rough crank pin portions.

The stationary journal dies are disposed at locations of the roughjournal portions connecting with the third and fourth rough crank armportions, configured to hold and retain such rough journal portionstherebetween in the direction perpendicular to the axial direction, andconfigured to be constrained from moving axially while being in contactwith side surfaces of the third and fourth rough crank arm portions.

The movable journal dies are disposed at locations of the correspondingrough journal portion excluding the rough journal portions connectingwith the third and fourth rough crank arm portions, configured to holdand retain such rough journal portions therebetween in the directionperpendicular to the axial direction, and configured to move axiallytoward the reference crank pin die while being in contact with sidesurfaces of the rough crank arm portions through which the rough crankarm portions connect with such rough journal portions.

The forming apparatus is configured such that in a state that the roughjournal portions are held and retained by the stationary and movablejournal dies and the rough crank pin portions are contacted with thereference crank pin die and the movable crank pin dies, the movablejournal dies are moved axially, the movable crank pin dies are movedaxially and in the direction perpendicular to the axial direction, andthe reference crank pin die is moved in the direction perpendicular tothe axial direction, thereby compressing the weighted rough crank armportions in the axial direction so as to reduce the thickness thereof tothe thickness of weighted crank arms of the forged crankshaft, andpressing the rough crank pin portions in the direction perpendicular tothe axial direction so as to increase the amount of eccentricity thereofto the amount of eccentricity of the crank pins of the forgedcrankshaft.

In the above forming apparatus in (1), it is preferred that thereference crank pin die and the movable crank pin dies each includes anauxiliary crank pin die disposed at a location outside of thecorresponding rough crank pin portion, opposite to the side where thereference crank pin die and the movable crank pin dies are contacted,and in conjunction with the axial movement of the movable journal diesas well as that of the movable crank pin dies and the auxiliary crankpin dies forming pairs therewith, a movement of the movable crank pindies in the direction perpendicular to the axial direction is controlledin a manner that the rough crank pin portions to be deformed by pressingreach to the auxiliary crank pin dies after spaces between the movablejournal dies, the reference crank pin die, the movable crank pin dies,and the auxiliary crank pin dies are filled.

This forming apparatus preferably has a configuration such that,provided that a total length of movement of the movable crank pin diesin the direction perpendicular to the axial direction is a 100% lengthof movement thereof, when the axial movement of the movable journal diesthat are adjacent to such movable crank pin dies is completed, a lengthof movement of such movable crank pin dies in the directionperpendicular to the axial direction is 90% or less of the total lengthof movement, and thereafter, the movement of such movable crank pin diesin the direction perpendicular to the axial direction is completed.

Further, the above forming apparatus in (1) may have a configurationsuch that the reference crank pin die, the movable crank pin dies, andthe stationary and movable journal dies are mounted on a press machinethat is capable of being moved downward along the directionperpendicular to the axial direction and, by the downward movement ofthe press machine, the stationary and movable journal dies are caused tohold and retain the rough journal portions therebetween while thereference crank pin die and the movable crank pin dies are brought intocontact with the rough crank pin portions, and with continued downwardmovement of the press machine, the movable journal dies are movedaxially by wedge mechanisms, and the movable crank pin dies are causedto move axially by the movement of the movable journal dies.

In case of this forming apparatus, it is preferred that the wedgemechanisms have different wedge angles for each movable journal die.Furthermore, it is preferred that the reference crank pin die and themovable crank pin dies are coupled to hydraulic cylinders and caused tomove in the direction perpendicular to the axial direction by drivingthe hydraulic cylinders.

(2) First and third rough crank pin portions at opposite ends in thepreform blank have an amount of eccentricity in a directionperpendicular to an axial direction in the opposite direction to eachother, the amount of eccentricity thereof being less than a √3/2 of anamount of eccentricity of the crank pins of the forged crankshaft, and asecond rough crank pin portion in the center in a preform blank has anamount of eccentricity in the direction perpendicular to the axialdirection of zero or has the same amount of eccentricity in a directionperpendicular to an eccentric direction of the first and third roughcrank pin portions as an amount of eccentricity of the crank pin of theforged crankshaft.

The forming apparatus includes a reference crank pin die, movable crankpin dies, and journal dies, described below.

The reference crank pin die is disposed at a location of the secondrough crank pin portion, configured to be brought into contact with thesecond rough crank pin portion, and configured to be constrained frommoving in the axial direction while, being in contact with side surfacesof the third and fourth rough crank arm portions through which the roughcrank arm portions connect with the second rough crank pin portion.

The movable crank pin dies are disposed at locations of thecorresponding first and third rough crank pin portions, configured to bebrought into contact with the first and third rough crank pin portions,and configured to move axially toward the reference crank pin die and inthe direction perpendicular to the axial direction, while being incontact with side surfaces of the rough crank arm portions through whichthe rough crank arm portions connect with the first and third roughcrank pin portions.

The stationary journal dies are disposed at locations of the roughjournal portions connecting with the third and fourth rough crank armportions, configured to hold and retain such rough journal portionstherebetween in the direction perpendicular to the axial direction, andconfigured to be constrained from moving axially while being in contactwith side surfaces of the third and fourth rough crank arm portions.

The movable journal dies are disposed at locations of the correspondingrough journal portion excluding the rough journal portions connectingwith the third and fourth rough journal portions, configured to hold andretain such rough journal portions therebetween in the directionperpendicular to the axial direction, and configured to move axiallytoward the reference crank pin die while being in contact with sidesurfaces of the rough crank arm portions through which the rough crankarm portions connect with such rough journal portions.

The forming apparatus in a state that the rough journal portions areheld and retained by the stationary and movable journal dies and therough crank pin portions are contacted with the reference crank pin dieand the movable crank pin dies, the movable journal dies are movedaxially and the movable crank pin dies are moved axially and in thedirection perpendicular to the axial direction, thereby compressing theweighted rough crank arm portions in the axial direction so as to reducethe thickness thereof to the thickness of weighted crank arms of aforged crankshaft, and pressing the first and third rough crank pinportions in the direction perpendicular to the axial direction, but inthe opposite direction to each other, so as to increase the amount ofeccentricity thereof to the √3/2 of the amount of eccentricity of crankpins of the forged crankshaft.

The manufacturing method according to embodiments of the presentinvention is a method for manufacturing a forged crankshaft for athree-cylinder engine, and includes any one of configurations (3) to (6)described below.

(3) A method for manufacturing includes the following successive stepscomprising a first preforming step, a second preforming step, and afinish forging step.

The first preforming step forms a preform blank to be supplied to theabove forming apparatus in (1). First and third rough crank pin portionsat opposite ends in a preform blank have an amount of eccentricity in adirection perpendicular to an axial direction in the opposite directionto each other, the amount of eccentricity thereof being equal to a √3/2of an amount of eccentricity of crank pins of the forged crankshaft. Asecond rough crank pin portion in the center in a preform blank has asmaller amount of eccentricity in the direction perpendicular to theaxial direction in the direction perpendicular to an eccentric directionof the first and third rough crank pin portions than an amount ofeccentricity of the crank pin of the forged crankshaft.

The second preforming step forms, using the above forming apparatusdescribed in (1), a blank for finish forging. The blank for finishforging has the final shape of the forged crankshaft including theplacement angle of the crank pins.

In the finish forging step, finish forging is performed on the blank forfinish forging to form a forged product having the final shape of theforged crankshaft including the placement angle of the crank pins.

(4) A method for manufacturing includes the following successive stepscomprising a first preforming step, a second preforming step, a finishforging step, and a twisting step.

The first preforming step forms a preform blank to be supplied to theabove forming apparatus in (1). First and third rough crank pin portionsat opposite ends in a preform blank have a smaller amount ofeccentricity in a direction perpendicular to an axial direction in thesame direction than an amount of eccentricity of crank pins of theforged crankshaft. A second rough crank pin portion in the center in apreform blank has a smaller amount of eccentricity in the directionperpendicular to the axial direction in the direction opposite to aneccentric direction of the first and third rough crank pin portions thanan amount of eccentricity of the crank pin of the forged crankshaft.

The second preforming step forms a blank for finish forging using theabove forming apparatus described in (1). The blank for finish forginghas the final shape of the forged crankshaft excluding the placementangle of the crank pins.

In the finish forging step, finish forging is performed on the blank forfinish forging to form a forged product having the final shape of theforged crankshaft excluding the placement angle of the crank pins.

In the twisting step, the placement angle of the crank pins of theforged product is adjusted to the placement angle of the crank pins ofthe forged crankshaft.

(5) A method for manufacturing includes the following successive stepscomprising a first preforming step, a second preforming step, and afinish forging step.

The first preforming step forms a preform blank to be supplied to theabove forming apparatus in (2). First and third rough crank pin portionsat opposite ends in a preform blank have an amount of eccentricity in adirection perpendicular to an axial direction in the opposite directionto each other, the amount of eccentricity thereof being less than a √3/2of an amount of eccentricity of the crank pins of the forged crankshaft.A second rough crank pin portion in the center in a preform blank has anamount of eccentricity in the direction perpendicular to the axialdirection of zero.

The second preforming step forms, using the above forming apparatusdescribed in (2), a blank for finish forging. The first and third roughcrank pin portions at opposite ends in a blank for finish forging havean amount of eccentricity in the direction perpendicular to the axialdirection in the opposite direction to each other, the amount ofeccentricity thereof being equal to the √3/2 of the amount ofeccentricity of the crank pins of the forged crankshaft. The secondrough crank pin portion in the center in a blank for finish forgingremains the same amount of eccentricity in the direction perpendicularto the axial direction as the preform blank.

In the finish forging step, finish forging is performed on the blank forfinish forging in a state that the first and third rough crank pinportions at opposite ends are horizontally placed, whereby all the roughcrank pin portions are pressed in the direction perpendicular to theaxial direction to form a forged product having a final shape of theforged crankshaft including the placement angle of the crank pins.

(6) A method for manufacturing includes the following successive stepscomprising a first preforming step, a second preforming step, and afinish forging step.

The first preforming step forms a preform blank to be supplied to theabove forming apparatus in (2). First and third rough crank pin portionsat opposite ends in a preform blank have an amount of eccentricity in adirection perpendicular to an axial direction in the opposite directionto each other, the amount of eccentricity thereof being less than a √3/2of an amount of eccentricity of the crank pins of the forged crankshaft.A second rough crank pin portion in the center in a preform blank has anamount of eccentricity in the direction perpendicular to the axialdirection in the direction perpendicular to an eccentric direction ofthe first and third rough crank pin portions, the amount of eccentricitythereof being the same as an amount of eccentricity of the crank pin ofthe forged crankshaft.

The second preforming step forms a blank for finish forging using theabove forming apparatus described in (2). The first and third roughcrank pin portions at opposite ends in a blank for finish forging havean amount of eccentricity in the direction perpendicular to the axialdirection in the opposite direction to each other, the amount ofeccentricity thereof being equal to the √3/2 of the amount ofeccentricity of the crank pins of the forged crankshaft. The secondrough crank pin portion in the center in a blank for finish forgingremains the same amount of eccentricity in the direction perpendicularto the axial direction as the preform blank.

In the finish forging step, finish forging is performed on the blank forfinish forging in a state that the first and third rough crank pinportions at opposite ends are horizontally placed, whereby the first andthird rough crank pin portions are pressed in the directionperpendicular to the axial direction to form a forged product having afinal shape of the forged crankshaft including the placement angle ofthe crank pins.

Advantageous Effects of Invention

With the forming apparatus of the present embodiment and themanufacturing method including the preforming steps in which suchapparatus is used, it is possible to form, from a preform blank withouta flash, a blank for finish forging without a flash which has a shapegenerally in agreement with a shape of a forged crankshaft for athree-cylinder engine having thin arms, even with the weighted arms. Bysubjecting such a blank for finish forging without a flash to finishforging, it is possible to obtain the final shape of the forgedcrankshaft including the contour shape of crank arms although some minoramount of flash is generated. Thus, forged crankshafts forthree-cylinder engines can be manufactured with high materialutilization and also with high dimensional accuracy regardless of theirshapes.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating a typical conventionalprocess for manufacturing a forged crankshaft for a three-cylinderengine.

FIG. 2 is a diagram schematically showing the shapes of a preform blankto be processed by the forming apparatus, a blank for finish forgingformed therefrom, and a forged product after finish forging, in themanufacturing method of a first embodiment.

FIG. 3 is a schematic diagram illustrating a process for manufacturing aforged crankshaft according to the first embodiment.

FIG. 4 is a longitudinal sectional view showing a configuration of theforming apparatus according to the first embodiment.

FIG. 5A is a longitudinal sectional view illustrating a process forforming a blank for finish forging using the forming apparatus accordingto the first embodiment shown in FIG. 4, with a forming state at aninitial stage shown therein.

FIG. 5B is a longitudinal sectional view illustrating a process forforming a blank for finish forging using the forming apparatus accordingto the first embodiment shown in FIG. 4, with a forming state at thecompletion shown therein.

FIG. 6 is a diagram illustrating how fin flaws occur in forming a blankfor finish forging using the forming apparatus.

FIG. 7 is a diagram illustrating how fin flaws are prevented by taking ameasure in forming a blank for finish forging using the formingapparatus.

FIG. 8 is a diagram schematically showing the shapes of a preform blankto be processed by the forming apparatus, a blank for finish forgingformed therefrom, a forged product after finish forging, and a twistedproduct after twisting, in the manufacturing method of a secondembodiment.

FIG. 9 is a schematic diagram illustrating a process for manufacturing aforged crankshaft according to the second embodiment.

FIG. 10 is a longitudinal sectional view showing a configuration of theforming apparatus according to the second embodiment.

FIG. 11A is a longitudinal sectional view illustrating a process forforming a blank for finish forging using the forming apparatus accordingto the second embodiment shown in FIG. 10, with a forming state at aninitial stage shown therein.

FIG. 11B is a longitudinal sectional view illustrating a process forforming a blank for finish forging using the forming apparatus accordingto the second embodiment shown in FIG. 10, with a forming state at thecompletion shown therein.

FIG. 12 is a diagram schematically showing the shapes of a preform blankto be processed by the forming apparatus, a blank for finish forgingformed therefrom, and a forged product after finish forging, in themanufacturing method of a third embodiment.

FIG. 13 is a schematic diagram illustrating a process for manufacturinga forged crankshaft according to the third embodiment.

FIG. 14 is a longitudinal sectional view showing a configuration of theforming apparatus according to the third embodiment.

FIG. 15A is a longitudinal sectional view illustrating a process forforming a blank for finish forging using the forming apparatus accordingto the third embodiment shown in FIG. 14, with a forming state at aninitial stage shown therein.

FIG. 15B is a longitudinal sectional view illustrating a process forforming a blank for finish forging using the forming apparatus accordingto the third embodiment shown in FIG. 14, with a forming state at thecompletion shown therein.

FIG. 16 is a diagram schematically showing the shapes of a preform blankto be processed by the forming apparatus, a blank for finish forgingformed therefrom, and a forged product after finish forging, in themanufacturing method of a fourth embodiment.

FIG. 17 is a schematic diagram illustrating a process for manufacturinga forged crankshaft according to the fourth embodiment.

DESCRIPTION OF EMBODIMENTS

The present invention is based on the premise that, in manufacturing aforged crankshaft for a three-cylinder engine, finish forging isperformed in the manufacturing process. The forming apparatus of thepresent invention is used for forming, in a step prior to finishforging, a blank for finish forging to be subjected to the finishforging, from a preform blank. With regard to the apparatus for forminga blank for finish forging for a forged crankshaft for a three-cylinderengine and the method for manufacturing a forged crankshaft for athree-cylinder engine including the preforming steps using suchapparatus, of the present invention, embodiments thereof are describedin detail below.

1. First Embodiment

1-1. Preform Blank, Blank for Finish Forging, and Forged Product

FIG. 2 is a diagram schematically showing the shapes of a preform blankto be processed by the forming apparatus, a blank for finish forgingformed therefrom, and a forged product after finish forging, in themanufacturing method of the first embodiment. FIG. 2 shows how athree-cylinder four-counterweight crankshaft is manufactured as anexample. In addition, FIG. 2 shows plane views showing an outsideappearance of the crankshaft and drawings depicting an arrangement ofcrank pins with a view along an axial direction side by side tofacilitate understanding of the shapes of the blanks in each step.

As shown in FIG. 2, a preform blank 4 of the first embodiment has acrankshaft shape that is approximate to a shape of a forged crankshaft 1for a three-cylinder four-counterweight shown in FIG. 1 (f) but isgenerally in a rough shape. The preform blank 4 includes: four roughjournal portions J1 a to J4 a; three rough crank pin portions P1 a to P3a; a rough front part portion Fra; a rough flange portion Fla; and sixrough crank arm portions A1 a to A6 a (hereinafter also referred tosimply as “rough arm portions”) that alternatively connect the roughjournal portions J1 a to J4 a, and the rough crank pin portions P1 a toP3 a to each other. The third and fourth rough arm portions A3 a and A4a connecting with the second rough crank pin portion P2 a in the centerhave no balance weight, therefore having oval shapes. The preform blank4 has no flash. Hereinafter, when the rough journal portions J1 a to J4a, the rough crank pin portions P1 a to P3 a, and the rough arm portionsA1 a to A6 a, of the preform blank 4, are each collectively referred to,a reference character “Ja” is used for the rough journal portions, areference character “Pa” for the rough crank pin portions, and areference character “Aa” for the rough arm portions. The first, second,fifth, and sixth rough arm portions A1 a, A2 a, A5 a, and A6 a havingbalance weights are also referred to as weighted rough arm portions Aa.On the other hand, the third and fourth rough arm portions A3 a and A4 ahaving no balance weight are also referred to as non-weighted rough armportions Aa, or oval rough arm portions Aa.

A blank for finish forging 5 of the first embodiment is formed from thepreform blank 4 described above using a forming apparatus, details ofwhich will be provided later. The blank for finish forging 5 includesfour rough journal portions J1 b to J4 b, three rough crank pin portionsP1 b to P3 b, a rough front part portion Frb, a rough flange portionFlb, and six rough crank arm portions A1 b to A6 b (hereinafter alsoreferred to simply as “rough arm portions”) that alternatively connectthe rough journal portions J1 b to J4 b, and the rough crank pinportions P1 b to P3 b to each other. The third and fourth rough armportions A3 b and A4 b connecting with the second rough crank pinportion P2 b in the center have no balance weight, therefore having ovalshapes. The blank for finish forging 5 has no flash. Hereinafter, whenthe rough journal portions J1 b to J4 b, the rough crank pin portions P1b to P3 b, and the rough arm portions A1 b to A6 b, of the blank forfinish forging 5, are each collectively referred to, a referencecharacter “Jb” is used for the rough journal portions, a referencecharacter “Pb” for the rough crank pin portions, and a referencecharacter “Ab” for the rough arm portions. The first, second, fifth, andsixth rough arm portions A1 b, A2 b, A5 b, and A6 b having balanceweights are also referred to as weighted rough arm portions Ab. On theother hand, the third and fourth rough arm portions A3 b and A4 b havingno balance weight are also referred to as non-weighted rough armportions Ab, or oval rough arm portions Ab.

A forged product 6 of the first embodiment is obtained from the blankfor finish forging 5 described above by finish forging. The forgedproduct 6 includes four journals J1 c to J4 c, three crank pins P1 c toP3 c, a front part Frc, a flange Flc, and six crank arms A1 c to A6 c(hereinafter also referred to simply as “arms”) that alternativelyconnect the journals J1 c to J4 c, and the crank pins P1 c to P3 c toeach other. The third and fourth arms A3 c and A4 c connecting with thesecond rough crank pin portion P2 c in the center have no balanceweight, therefore having oval shapes. Hereinafter, when the journals J1c to J4 c, the crank pins P1 c to P3 c, and the arms A1 c to A6 c, ofthe forged product 6, are each collectively referred to, a referencecharacter “Jc” is used for the journals, a reference character “Pc” forthe crank pins, and a reference character “Ac” for the arms. The first,second, fifth, and sixth arms A1 c, A2 c, A5 c, and A6 c having balanceweights are also referred to as weighted arms Ac. On the other hand, thethird and fourth arms A3 c and A4 c having no balance weight are alsoreferred to as non-weighted arms Ac, or oval arms Ac.

The forged product 6 has a shape that is in agreement with a shape of acrankshaft (forged final product) including a placement angle of thecrank pins Pc and corresponds to a forged crankshaft 1 shown in FIG.1(f). Specifically, the journals Jc of the forged product 6 have anaxial length equal to that of journals J of the forged crankshaft havingthe final shape. The crank pins Pc of the forged product 6 have an axiallength equal to that of crank pins P of the forged crankshaft having thefinal shape. Further, the crank pins Pc of the forged product 6 have thesame amount of eccentricity in a direction perpendicular to an axialdirection and the same placement angle of 120° as the crank pins P ofthe forged crankshaft having the final shape, thus they are placed atthe specified positions. The arms Ac of the forged product 6 have anaxial thickness equal to that of arms A of the forged crankshaft havingthe final shape.

The blank for finish forging 5 has a shape that is generally inagreement with the shape of the forged product 6 and corresponds exactlyto a block forged blank 105 shown in FIG. 1(d) with a differencetherebetween being a flash 105 a. Specifically, the rough journalportions Jb of the blank for finish forging 5 have an axial length equalto that of the journals J of the forged crankshaft having the finalshape (journals Jc of forged product 6). The rough crank pin portions Pbof the blank for finish forging 5 have an axial length equal to that ofthe crank pins P of the forged crankshaft having the final shape (crankpins Pc of forged product 6). Further, the crank pins Pb of the blankfor finish forging 5 have the same amount of eccentricity in thedirection perpendicular to the axial direction and the same placementangle of 120° as the crank pins P of the forged crankshaft having thefinal shape, thus they are placed at the specified positions. The rougharm portions Ab of the blank for finish forging 5 have an axialthickness equal to that of the arms A of the forged crankshaft havingthe final shape (arms Ac of forged product 6).

In contrast, the rough journal portions Ja of the preform blank 4 havean axial length equal to that of the rough journal portions Jb of theblank for finish forging 5, i.e., that of the journals J of the forgedcrankshaft (journals Jc of forged product 6). The rough crank pinportions Pa of the preform blank 4 have an axial length equal to that ofthe rough crank pin portions Pb of the blank for finish forging 5, i.e.,that of the crank pins P of the forged crankshaft (crank pins Pc offorged product 6), but have a smaller amount of eccentricity than thatof the rough crank pin portions Pb of the blank for finish forging 5.Specifically, the first and third rough crank pin portions P1 a and P3 aat opposite ends among the rough crank pin portions Pa of the preformblank 4 have an amount of eccentricity in the opposite direction to eachother, the amount of eccentricity thereof being equal to a √3/2 of anamount of eccentricity in the crank pins P of the forged crankshaft. Onthe other hand, the second rough crank pin portion P2 a in the center isconfigured to have an amount of eccentricity in the directionperpendicular to an eccentric direction of the first and third roughcrank pin portions P1 a and P3 a, the amount of eccentricity thereofbeing approximately equal to a half of an amount of eccentricity in thecrank pin P of the forged crankshaft.

Among the rough arm portions Aa of the preform blank 4, the weightedrough arm portions Aa (the first, second, fifth and sixth rough armportions A1 a, A2 a, A5 a, and A6 a) have an axial thickness greaterthan that of the respective weighted rough arm portions Ab of the blankfor finish forging 5, i.e., weighted arms A of the forged crankshaft(the weighted arms Ac of the forged product 6). On the other hand, theoval rough arm portions Aa of the preform blank 4 (the third and fourthrough arm portions A3 a and A4 a) have an axial thickness equal to thatof the respective oval rough arm portions Ab of the blank for finishforging 5, i.e., the oval arms A (the oval arm Ac of the forged product6) of the forged crankshaft. In brief, compared to the blank for finishforging 5 (the forged crankshaft having the final shape and the forgedproduct 6), the preform blank 4 has an overall length that is relativelylong by the additional thickness of the weighted rough arm portions Aa,and has a relatively small amount of eccentricity of the rough crank pinportions Pa. Thus, the preform blank 4 has a relatively gentlecrankshaft shape.

However, strictly speaking, the blank for finish forging 5 has such aconfiguration that, with respect to the final shapes of the forgedcrankshaft and the forged product 6, the rough arm portions Ab are madeslightly thinner and therefore the axial lengths of the rough journalportions Jb and the rough crank pin portions Pb are accordingly slightlygreater. This is intended to ensure that the blank for finish forging 5can be easily received by the dies when finish forging is performed andthereby prevent the occurrence of scoring. Correspondingly, the preformblank 4, too, has such a configuration that, with respect to the finalshapes of the forged crankshaft and the forged product 6, the axiallengths of the rough journal portions Ja and the rough crank pinportions Pa are accordingly slightly greater.

1-2. Process for Manufacturing Forged Crankshaft

FIG. 3 is a schematic diagram illustrating a process for manufacturing aforged crankshaft according to the first embodiment. As shown in FIG. 3,the process for manufacturing the forged crankshaft for thethree-cylinder engine of the first embodiment includes a firstpreforming step, a second preforming step, and a finish forging step,and also includes a trimming step and a coining step as necessary.

The first preforming step is a step in which the preform blank 4described above is obtained. In the first preforming step, by using around billet having a circular cross section as a starting material, apreforming operation is applied to the round billet after it is heatedby an induction heater or a gas atmosphere furnace. For example, theround billet is subjected to roll forming in which it isreduction-rolled by grooved rolls to distribute its volume in thelongitudinal direction; and the resulting rolled blank is repeatedlysubjected to bending in which it is partially pressed in a press from adirection perpendicular to the longitudinal direction to distribute itsvolume. In this manner, the preform blank 4 can be obtained. Also, thepreform blank 4 may be obtained by using the techniques disclosed inPatent Literatures 1 and 2. Furthermore, cross roll forging,fully-enclosed die forging or the like may also be employed.

The second preforming step is a step in which the blank for finishforging 5 described above is obtained. In the second preforming step,operation is applied by using a forming apparatus described in FIG. 4below. In this manner, the blank for finish forging 5 having the finalshape of the forged crankshaft including the placement angle of crankpins can be obtained from the preform blank 4 described above.

The finish forging step is a step in which the forged product 6described above is obtained. In the finish forging step, the blank forfinish forging 5 is supplied to be processed by press forging with apair of upper and lower dies, whereby the forged product 6 having ashape in agreement with the shape of the crankshaft of the forgedcrankshaft having the final shape including the placement angle of thecrank pins can be obtained.

1-3. Apparatus for Forming Blank for Finish Forging

FIG. 4 is a longitudinal sectional view showing a configuration of theforming apparatus according to the first embodiment. FIG. 4 illustrates,as an example, a forming apparatus that is used in manufacturing athree-cylinder four-counterweight crankshaft, i.e., a forming apparatusconfigured to form the blank for finish forging 5 from the preform blank4 shown in FIG. 2. It should be noted that in the longitudinal sectionalview shown in FIG. 4, the first and third rough crank pin portions arein reality extended in a front-back direction, where either one of themis located in the front side of the paper and the other one is locatedin the back side of the paper, however they are illustrated on the sameplane for convenience.

As shown in FIG. 4, the forming apparatus is configured to utilize apress machine and includes a stationary lower pressure pad 20 serving asa base and an upper pressure pad 21, which is lowered by driving a ramof the press machine. A lower die holder 22, located over the lowerpressure pad 20, is resiliently supported via a resilient member 24.This lower die holder 22 is vertically movable. As the resilient member24, disc springs, coil springs, air springs, or the like may beemployed, or a hydraulic spring system may be employed. An upper dieholder 23 is secured under the upper pressure pad 21 via support posts25. This upper die holder 23 is lowered together with the upper pressurepad 21 by driving the press machine (ram).

In the forming apparatus shown in FIG. 4, the preform blank 4 is placedin the dies in a manner such that the first and third rough crank pinportions P1 a and P3 a are horizontally positioned and the second roughcrank pin portion P2 a is positioned in a lower side in the verticaldirection. The preform blank 4 placed in such a manner is formed intothe blank for finish forging. Thus, vertically forming pairs, i.e., thestationary journal dies 9U and 9B, the movable journal dies 10U and 10B,the reference crank pin die 11 and the auxiliary crank pin die 13, andthe movable crank pin dies 12 and the auxiliary crank pin dies 13, areapart from each other in the axial direction of the preform blank 4, andthe lower and upper ones are respectively mounted on the lower dieholder 22 and the upper die holder 23.

The reference crank pin die 11 and the auxiliary crank pin die 13,vertically forming a pair, are disposed serving as a reference among therough crank pin portions Pa of the preform blank 4, the location of thesecond rough crank pin portion P2 a in the center, with the upper onemounted on the upper die holder 23 and the lower one mounted on thelower die holder 22. The reference crank pin die 1 of the firstembodiment is disposed on the opposite side of a specified position ofsecond rough crank pin portions P2 a, whereas its counterpart, theauxiliary crank pin die 13 is disposed in the same side of the specifiedposition of the second rough crank pin portion P2 a in the outside. Forexample, at the location of the second rough crank pin portion P2 a, thesecond rough crank pin portion P2 a is positioned in the lower side.Thus the specified position thereof is located in the lower side, as aresult, the reference crank pin die 11 is mounted on the upper dieholder 23, and its counterpart, the auxiliary crank pin die 13 ismounted on the lower die holder 22.

Particularly, the reference crank pin die 11 and the auxiliary crank pindie 13, i.e., both the upper and lower dies, are constrained from movingin the axial direction on the upper die holder 23 and the lower dieholder 22, respectively. Only the reference crank pin die 11 is movablein the direction perpendicular to the axial direction, i.e., thedirection toward the specified position of the rough crank pin portionPa (downward direction in FIG. 4).

The reference crank pin die 11 and the auxiliary crank pin die 13respectively have impressions 11 a and 13 a having a semi-cylindricalshape. The length of the impressions 11 a and 13 a is equal to the axiallength of the rough crank pin portion P2 b of the blank for finishforging 5.

By the lowering of the upper die holder 23 caused by driving the pressmachine, i.e., the downward movement of the press machine, the referencecrank pin die 11 is brought into a state in which the impression 11 a isbrought into contact with the second rough crank pin portion P2 a.Concurrently, both side surfaces of the reference crank pin die 11 arein contact with the third and fourth rough arm portions A3 a and A4 a ata second rough crank pin portion P2 a-side side surfaces through whichthe third and fourth rough arm portions A3 a and A4 a and the secondrough crank pin portion P2 a are connected.

The movable crank pin dies 12 and the auxiliary crank pin dies 13,vertically forming pairs with each other, are disposed is in contact,the locations of the first and third rough crank pin portions P1 a andP3 a, with the upper ones mounted on the upper die holder 23 and thelower ones mounted on the lower die holder 22. The movable crank pindies 12 of the first embodiment are disposed on the opposite side ofspecified positions of the corresponding rough crank pin portions Pa,whereas their counterparts, the auxiliary crank pin dies 13 are disposedon the same side of the specified positions of the corresponding roughcrank pin portions Pa in the outside. For example, at the location ofthe first rough crank pin portion P1 a, the specified position of thefirst rough crank pin portion P1 a is located in the upper side. Thusthe corresponding movable crank pin die 12 is mounted on the lower dieholder 22, and its counterpart, the auxiliary crank pin die 13 ismounted on the upper die holder 23.

Particularly, all the movable crank pin dies 12 and the auxiliary crankpin dies 13, i.e., both the upper and lower dies, are axially movabletoward the reference crank pin die 11 on the lower die holder 22 and theupper die holder 23, respectively. Only the movable crank pin dies 12are movable in the direction perpendicular to the axial direction, i.e.,the direction toward the specified positions of the rough crank pinportions Pa (upward direction in FIG. 4).

The movable crank pin dies 12 and the auxiliary crank pin dies 13respectively have impressions 12 a and 13 a having a semi-cylindricalshape. The length of the impressions 12 a and 13 a is equal to the axiallength of the rough crank pin portions Pb of the blank for finishforging 5.

The stationary journal dies 9U and 9B are disposed at locations of thecorresponding rough journal portion Ja of the preform blank 4 (thesecond and third rough journal portions J2 a and J3 a) connecting withthe rough oval arm portion Aa (the third and fourth rough arm portionsA3 a and A4 a), with the upper ones mounted on the upper die holder 23and the lower ones mounted on the lower die holder 22. Particularly, thestationary journal dies 9U and 9B, i.e., both the upper and lower ones,are completely secured to the upper die holder 23 and the lower dieholder 22, respectively, and constrained from moving in the axialdirection.

The stationary journal dies 9U and 9B have first impressions 9Ua and9Ba, respectively, each having a semi-cylindrical shape and secondimpressions 9Ub and 9Bb, and third impressions 9Uc and 9Bc,respectively, each located at the front and back (right and left as seenin FIG. 4) the first impressions 9Ua and 9Ba and adjacent thereto. Thelength of the first impressions 9Ua and 9Ba is equal to the axial lengthof the rough journal portions Jb (the second and third rough journalportions J2 b and J3 b) of the blank for finish forging 5. The length ofthe second impressions 9Ub and 9Bb is equal to the axial thickness ofthe weighted rough arm portions Ab (the second and fifth rough armportions A2 b and A5 b) connecting to the rough journal portions Jb ofthe blank for finish forging 5. The length of the third impressions 9Ucand 9Bc is equal to the axial thickness of the rough oval arm portionsAb (the third and fourth rough arm portion A3 b and A4 b) connecting tothe rough journal portions Jb of the blank for finish forging 5.

By the downward movement of the press machine, the stationary journaldies 9U and 9B are caused to hold and retain the rough journal portionsJa with the corresponding first impressions 9Ua and 9Ba therebetweenfrom above and below. Concurrently, the stationary journal dies 9U and9B are brought into a state in which the second impressions 9Ub and 9Bband the third impressions 9Uc and 9Bc, at their first impressions 9Uaand 9Ba-side surfaces, are in contact with the weighted rough armportions Aa and the rough oval arm portions Aa connecting with thecorresponding rough journal portions Ja, at their rough journal portionJa-side side surfaces.

The movable journal dies 10U and 10B are disposed at locations of thecorresponding the rough journal portions Ja (first and fourth roughjournal portions J1 a and J4 a) of the preform blank 4 excluding therough journal portions Ja (second and third rough journal portions J2 aand J3 a) connecting with the rough oval arm portions Aa (third andfourth rough arm portions A3 a and A4 a), with the upper ones mounted onthe upper die holder 23 and the lower ones mounted on the lower dieholder 22. Particularly, the movable journal dies 10U and 10B, i.e.,both the upper and lower dies, are axially movable toward the referencecrank pin die 11 to the upper die holder 23 and the lower die holder 22,respectively.

The movable journal dies 10U and 10B respectively have first impressions10Ua and 10Ba having a semi-cylindrical shape and respectively havesecond impressions 10Ub and 10Bb, located adjacent to the firstimpressions 10Ua and 10Ba at the front and back (right and left as seenin FIG. 4). The length of the first impressions 10Ua and 10Ba is equalto the axial length of the rough journal portions Jb (first and fourthjournal portions J1 b and J4 b) of the blank for finish forging 5. Thelength of the second impressions 10Ub and 10Bb is equal to the axialthickness of the weighted rough arm portions Ab (first and sixth rougharm portions A1 b and A6 b) connecting to the rough journal portions Jbof the blank for finish forging 5.

By the lowering of the upper die holder 23 caused by driving a pressmachine, i.e., the downward movement of the press machine, the movablejournal dies 10U and 10B are caused to hold and retain the respectiverough journal portions Ja from the upper and lower sides with the firstimpressions 10Ua and 10Ba. Concurrently, the movable journal dies 10Uand 10B are brought into a state in which the second impressions 10Uband 10Bb, at their first impression 10Ua and 10Ba-side surfaces, are incontact with the rough arm portions Aa, at their rough journal portionJa-side side surfaces through which the weighted rough arm portions Aaand the respective rough journal portions Ja are connected.

In this operation, by the downward movement of the press machine, thereference crank pin die 11 and the movable crank pin dies 12 are placedin a state in which the impressions 11 a and 12 a are brought intocontact with the rough crank pin portions Pa, and both side surfaces ofthe reference crank pin die 11 and the movable crank pin dies 12 are incontact with the rough arm portions Aa at their rough crank pin portionPa-side side surfaces through which the rough arm portions Aa and roughcrank pin portions Pa are connected.

The movable journal dies 10U and 10B disposed at locations of thecorresponding first and fourth rough journal portions J1 a and J4 a atopposite ends have end surfaces, which are respectively referred to asinclined surfaces 14U and 14B. In relation to this, on the lowerpressure pad 20, there are provided wedges 26, each locatedcorrespondingly to the location of the inclined surfaces 14U and 14B ofthe movable journal dies 10U and 10B for the first and fourth roughjournal portions J1 a and J4 a. Each of wedges 26 extends upwardpenetrating through the lower die holder 22. The inclined surfaces 14Bof the lower movable journal dies 10B, among the movable journal dies10U and 10B for the first and fourth rough journal portions J1 a and J4a, are in contact with the slopes of wedges 26 in the initial condition.On the other hand, the inclined surfaces 14U of the upper movablejournal dies 10U are brought into contact with the slopes of wedges 26by the downward movement of the press machine.

Then, with continued downward movement of the press machine, the uppermovable journal dies 10U are pressed downwardly together with the lowermovable journal dies 10B. This allows the movable journal dies 10U and10B for the first and fourth rough journal portions J1 a and J4 a, i.e.,both the upper and lower ones, to move axially toward the referencecrank pin die 11 for the second rough crank pin portion P2 a serving asa reference, as their inclined surfaces 14U and 14B slide along theslopes of the first wedges 26. Essentially, the movable journal dies 10Uand 10B are all capable of being moved axially by the wedge mechanisms.

Then, the movable crank pin dies 12 and the auxiliary crank pin dies 13are pressed downwardly together with continued downward movement of thepress machine. Accordingly, with the axial movement of the movablejournal dies 10U and 10B as described above, the movable crank pin dies12 and the auxiliary crank pin dies 13 are moved axially along with themtoward the reference crank pin die 11 for the second rough crank pinportion P2 a serving as a reference. The movement of the reference crankpin die 11 and the movable crank pin die 12 in the directionperpendicular to the axial direction is accomplished by driving thehydraulic cylinders 16 coupled to the crank pin dies 11 and 12.

It should be noted that the axial movement of the movable crank pin dies12 and the auxiliary crank pin dies 13 may be forcibly caused using awedge mechanism similar to the one for the movable journal dies 10U and10B or a separate mechanism such as a hydraulic cylinder or a servomotor or the like. The auxiliary crank pin dies 13 may be integral withone of their adjacent movable journal dies 10U and 10B or the stationaryjournal dies 9U and 9B forming pairs.

In the initial condition shown in FIG. 4, spaces are provided betweenthe axially arranged movable journal dies 10U and 10B and the stationaryjournal dies 9U and 9B, and their corresponding movable crank pin dies12 and auxiliary crank pin dies 13. The spaces are provided so as toallow the axial movement of the movable journal dies 10U and 10B as wellas that of the movable crank pin dies 12 and the auxiliary crank pindies 13. The size of the spaces represents the difference between thethickness of the rough arm portions Ab of the blank for finish forging 5and the thickness of the weighted rough arm portions Aa of the preformblank 4.

Next, descriptions are given as to how the blank for finish forging isformed using the thus configured forming apparatus.

FIG. 5A and FIG. 5B are longitudinal sectional views illustrating aprocess for forming a blank for finish forging using the formingapparatus of the first embodiment shown in FIG. 4. Among these figures,FIG. 5A shows a forming state at the initial stage and FIG. 5B shows aforming state at the completion.

The preform blank 4 is placed in the lower movable journal die 10B,stationary journal die 9B, the movable crank pin dies 12, and theauxiliary crank pin dies 13, shown in FIG. 4, and then lowering of thepress machine is started. Then, as shown in FIG. 5A, the upper movablejournal dies 10U and stationary journal dies 9U are brought into contactwith the corresponding lower movable journal dies 10B and stationaryjournal dies 9B.

Thus, the preform blank 4 is brought into a state in which the roughjournal portions Ja are held by the movable journal dies 10U and 10B andstationary journal dies 9U and 9B from above and below, and the roughcrank pin portions Pa are contacted by the reference crank pin die 11and the movable crank pin dies 12. In this state, in the preform blank4, the rough arm portions Aa, at their rough journal portion Ja-sideside surfaces, are in contact with the movable journal dies 10U and 10Band stationary journal dies 9U and 9B, and the rough arm portions Aa, attheir rough crank pin portion Pa-side side surfaces, are in contact withthe reference crank pin die 11 and the movable crank pin dies 12.Further, in this state, the inclined surfaces 14U and 14B of the movablejournal dies 10U and 10B for the first and fourth rough journal portionsJ1 a and J4 a are in contact with the slopes of wedges 26.

In this state, the lowering of the press machine is continued.Accordingly, the inclined surfaces 14U and 14B of the movable journaldies 10U and 10B for the first and fourth rough journal portions J1 aand J4 a slide along the slopes of the first wedges 26, and by thiswedge mechanism, these journal dies 10U and 10B are allowed to moveaxially toward the reference crank pin die 1 for the second rough crankpin portion P2 a. By such axial movement of the movable journal dies 10Uand 10B caused by the wedge mechanism, the movable crank pin dies 12 andthe auxiliary crank pin dies 13 are also allowed to move axially towardthe reference crank pin die 11.

Accordingly, the spaces between the movable journal dies 10U and 10B andstationary journal dies 9U and 9B, the movable crank pin dies 12, andthe auxiliary crank pin dies 13 are gradually narrowed, and finallyfilled. In this process, in the preform blank 4, the rough arm portionsAa are axially compressed by the movable journal dies 10U and 10B andstationary journal dies 9U and 9B, the reference crank pin die 11, andthe movable crank pin dies 12, while the axial lengths of the roughjournal portions Ja and the rough crank pin portions Pa are maintained,so that the thickness of the weighted rough arm portions Aa is reducedto the thickness of the weighted rough arm portions Ab of the blank forfinish forging 5 (see FIG. 5B).

Also, in coordination with the axial movement of the movable journaldies 10U and 10B as well as that of the movable crank pin dies 12 andthe auxiliary crank pin dies 13, each of the hydraulic cylinders 16 forthe reference crank pin die 11 and the movable crank pin dies 12 isoperated. Accordingly, the crank pin dies 11 and 12 press thecorresponding rough crank pin portions Pa of the preform blank 4 in thedirection perpendicular to the axial direction. Thus, the rough crankpin portions Pa of the preform blank 4 are displaced in the verticaldirection perpendicular to the axial direction, and an amount ofeccentricity thereof is increased to an amount of eccentricity of therough crank pin portions Pb of the blank for finish forging 5, bringinginto a state in which all the rough crank pin portions Pb are disposedin their specified positions (see FIGS. 2 and 5B).

In this manner, it is possible to form, from the preform blank 4 withouta flash, the blank for finish forging 5 without a flash, which has ashape generally in agreement with the shape of the forged crankshaft forthe three-cylinder engine having thin arms A (forged final product),even with the weighted arms A. By supplying such a blank for finishforging 5 without a flash for finish forging, and performing finishforging with it, it is possible to obtain the final shape of the forgedcrankshaft for the three-cylinder engine including the contour shape ofarms and the placement angle of the crank pins, although some minoramount of flash is generated. Therefore, forged crankshafts forthree-cylinder engines can be manufactured with high materialutilization and also with high dimensional accuracy regardless of theirshapes.

In the forming apparatus shown in FIGS. 4, 5A and 5B, the inclinedsurfaces 14U and 14B of the movable journal dies 10U and 10B for thefirst rough journal portion J1 a and its contacting slope of wedge 26,and the inclined surfaces 14U and 14B of the movable journal dies 10Uand 10B for the fourth rough journal portion J4 a and its contactingslope of wedge 26 are angled in a reverse relationship relative to avertical plane. The purpose of varying, for each of the movable journaldies 10U and 10B, the wedge angle of the wedge mechanism, which causesthe axial movement of the movable journal dies 10U and 10B, is to ensurethat the rate of deformation at which the weighted rough arm portions Aaare axially compressed to reduce the thickness thereof stays constantfor all the weighted rough arm portions Aa.

In the preform blank 4, which is processed by the forming apparatusshown in FIGS. 4, 5A and 5B, the rough journal portions Ja have across-sectional area that is equal to or greater than that of the roughjournal portions Jb of the blank for finish forging 5, i.e., that of thejournals J of the forged crankshaft. Similarly, the rough crank pinportions Pa of the preform blank 4 have a cross-sectional area that isequal to or greater than that of the rough crank pin portions Pb of theblank for finish forging 5, i.e., that of the crank pins P of the forgedcrankshaft. Even when the cross-sectional area of the rough journalportions Ja of the preform blank 4 is greater than the cross-sectionalarea of the rough journal portions Jb of the blank for finish forging 5,it is possible to reduce the cross-sectional area of the rough journalportions Ja to the cross-sectional area of the rough journal portions Jbof the blank for finish forging 5. This is caused by the holding andretaining of the rough journal portions Ja by the movable journal dies10U and 10B and by the subsequent axial movement of the movable journaldies 10U and 10B. Even when the cross-sectional area of the rough crankpin portions Pa of the preform blank 4 is greater than thecross-sectional area of the rough crank pin portions Pb of the blank forfinish forging 5, the cross-sectional area of the rough crank pinportions Pa can be reduced to the cross-sectional area of the roughcrank pin portions Pb of the blank for finish forging 5. This is causedby, in addition to the movement of the reference crank pin die 11 in thedirection perpendicular to the axial direction, the movement of themovable crank pin dies 12 in the axial direction and the directionperpendicular thereto.

An issue to be addressed regarding the forming of the blank for finishforging described above is local formation of fin flaws against theweighted rough arm portion Aa. The following describes how fin flaws areformed and how they can be prevented.

FIG. 6 is a diagram illustrating how fin flaws occur in forming a blankfor finish forging using the forming apparatus, and FIG. 7 is a diagramillustrating how fin flaws are prevented by taking a measure. In FIGS. 6and 7, there are shown (a) a forming state at an initial stage, (b) aforming state during the process, (c) a forming state at the completion,and (d) a blank for finish forging, which is removed from the formingapparatus after the completion of forming.

As shown in FIG. 6(a), upon the start of the forming operation, themovable journal dies 10U and 10B move axially, and the movable crank pindies 12 and the auxiliary crank pin dies 13 move axially and in thedirection perpendicular to this direction. Then, as shown in FIG. 6(b),in the weighted rough arm portions Aa, if the rough crank pin portionsPa in the process of pressing deformation in the direction perpendicularto the axial direction reach the auxiliary crank pin dies 13 before thecompletion of the axial movement of the movable journal dies 10U and10B, and the movable crank pin dies 12 and the auxiliary crank pin dies13, i.e., before the spaces between the movable journal dies 10U and 10Band the stationary journal dies 9U and 9B, and their correspondingmovable crank pin dies 12 and auxiliary crank pin dies 13 are filled, aproblem to be described below will occur. The fillings of the roughcrank pin portions Pa flow into the spaces between the auxiliary crankpin dies 13 and the movable journal dies 10U and 10B and stationaryjournal dies 9U and 9B. Although the fillings that have flowed thereintoare thinned with the progress of the forming operation, they stay thereeven after the forming operation is completed as shown in FIG. 6(c).Thus, as shown in FIG. 6(d), fin flaws 5 a, coming out of the roughcrank pin portions Pb of the blank for finish forging 5, are formedlocally at the boundaries with adjacent weighted rough arm portions Aa.

In the subsequent finish forging step, the fin flaws 5 a will be struckinto the finished product, resulting in causing overlaps. Therefore, inorder to ensure product quality, it is necessary to prevent theformation of the fin flaws.

One measure to prevent the formation of the fin flaws may be to controlthe movement of the movable crank pin dies 12 in the directionperpendicular to the axial direction so that the rough crank pinportions Pa to be processed for deformation by pressing reach theauxiliary crank pin dies 13 after the spaces between the movable journaldies 10U and 10B and stationary journal dies 9U and 9B, the movablecrank pin dies 12, and the auxiliary crank pin dies 13 in weighted rougharm portions Aa, are filled. Specifically, it may be configured suchthat the axial movement of the movable journal dies 10U and 10B as wellas that of the movable crank pin dies 12 and the auxiliary crank pindies 13 forming pair with the movable crank pin dies 12 is completed,thereafter the movement of the movable crank pin dies 12 in thedirection perpendicular to the axial direction is completed. Forexample, when the total moved distance of the movable crank pin dies 12in the direction perpendicular to the axial direction is designated as a100% moved distance thereof, it is preferred that, when the axialmovement of the movable journal dies 10U and 10B that are adjacent tothe movable crank pin dies 12 is completed, the moved distance of themovable crank pins die 12 in the direction perpendicular to the axialdirection is 90% or less (more preferably 83% or less, and even morepreferably 60% or less) of the total moved distance. Thereafter, themovement of the movable crank pin dies 12 in the same direction may becompleted.

For example, the forming operation is started as shown in FIG. 7(a). Andthen, as shown in FIG. 7(b), the axial movement of the movable journaldies 10U and 10B as well as that of the movable crank pin dies 12 andthe auxiliary crank pin dies 13 are completed before the length ofmovement of the movable crank pin dies 12 in the direction perpendicularto the axial direction reaches 90% of the total length of movement inweighted rough arm portions Aa. Consequently, by this time, the spacesbetween the movable journal dies 10U and 10B and stationary journal dies9U and 9B, the movable crank pin dies 12, and the auxiliary crank pindies 13 have been filled, whereas the rough crank pin portions Pa to beprocessed for deformation by pressing have not reached the auxiliarycrank pin dies 13. Subsequently, along with the movement of the movablecrank pin dies 12 in the direction perpendicular to the axial direction,the rough crank pin portions Pa reach the auxiliary crank pin dies 13,and with the completion of the movement, the forming is completed asshown in FIG. 7(c). Thus, no such problem occurs as the fillings of therough crank pin portions Pa flow into the spaces between the auxiliarycrank pin dies 13 and the movable journal dies 10U and 10B andstationary journal dies 9U and 9B. As a result, as shown in FIG. 7(d), ahigh quality blank for finish forging 5 without the fin flaws can beobtained.

The process of movement of the movable crank pin dies in the directionperpendicular to the axial direction before the completion of the axialmovement of the movable journal dies may be varied as desired. Forexample, the movement of the movable crank pin dies in the directionperpendicular to the axial direction may be started simultaneously withthe start of the axial movement of the movable journal dies or inadvance of that, or conversely, it may be started after the axialmovement of the movable journal dies has progressed to some extent.Also, the movement of the movable crank pin dies in the directionperpendicular to the axial direction may be stopped temporarily afterits start, at positions a certain distance away from their initialpositions, and it may be resumed after the completion of the axialmovement of the movable journal dies.

2. Second Embodiment

A second embodiment is based on the configuration of the firstembodiment described above. A second embodiment includes a twisting stepin a process of manufacturing a forged crankshaft as well asmodifications of the configuration related to this step.

2-1. Preform Blank, Blank for Finish Forging, Forged Product, andTwisted Product

FIG. 8 is a diagram schematically showing the shapes of a preform blankto be processed by the forming apparatus, a blank for finish forgingformed therefrom, a forged product after finish forging, and a twistedproduct after twisting, in the manufacturing method of the secondembodiment. FIG. 8 shows how a three-cylinder four-counterweightcrankshaft is manufactured, as seen in FIG. 2. This is also the case forthird and fourth embodiments described later.

As shown in FIG. 8, a preform blank 4 of the second embodiment has acrankshaft shape that is approximate to the shape of a forged crankshaft1 for a three-cylinder four-counterweight, but is generally in roughshape. The preform blank 4 includes four rough journal portions Ja,three rough crank pin portions Pa, a rough front part portion Fra, arough flange portion Fla, and six rough arm portions Aa. A blank forfinish forging 5 of the second embodiment is formed from the preformblank 4 described above using a forming apparatus, details of which willbe provided later. The blank for finish forging 5 includes four roughjournal portions Jb, three rough crank pin portions Pb, a rough frontpart portion Frb, a rough flange portion Flb, and six rough arm portionsAb. A forged product 6 of the second embodiment is obtained from theblank for finish forging 5 described above by finish forging. The forgedproduct 6 includes four journals Jc, three crank pins Pc, a front partFrc, a flange Flc, and six arms Ac.

A twisted product 7 of the second embodiment is obtained from the forgedproduct 6 described above by twisting. The twisted product 7 includesfour journals J1 d to J4 d, three crank pins P1 d to P3 d, a front partFrd, a flange Fld, and six crank arms A1 d to A6 d (hereinafter alsoreferred to simply as “arms”) that alternatively connect the journals J1d to J4 d, and the crank pins P1 d to P3 d to each other. Hereinafter,when the journals J1 d to J4 d, the crank pins P1 d to P3 d, and thearms A1 d to A6 d, of the twisted product 7, are each collectivelyreferred to, a reference character “Jd” is used for the journals, areference character “Pd” for the crank pins, and a reference character“Ad” for the arms.

The twisted product 7 has a shape that is in agreement with a shape of acrankshaft (forged final product) including a placement angle of thecrank pins Pd. Specifically, the journals Jd of the twisted product 7have an axial length equal to that of the journals J of the forgedcrankshaft having the final shape. The crank pins Pd of twisted product7 have an axial length equal to that of the crank pins P of the forgedcrankshaft having the final shape. Further, the crank pins Pd of thetwisted product 7 have the same amount of eccentricity in the directionperpendicular to the axial direction and the same placement angle of120° as the crank pins P of the forged crankshaft having the finalshape, thus they are placed at the specified positions. The arms Ad ofthe twisted product 7 have an axial thickness equal to that of arms A ofthe forged crankshaft having the final shape.

The forged product 6 has a shape that is in agreement with the shape ofthe crankshaft (forged final product) excluding the placement angle ofthe crank pins Pc. Specifically, the journals Jc of the forged product 6have an axial length equal to that of the journals J of the forgedcrankshaft having the final shape. The crank pins Pc of the forgedproduct 6 have an axial length equal to that of the crank pins P of theforged crankshaft having the final shape, and an amount of eccentricityin the direction perpendicular to the axial direction is the samebetween them. However, the placement angle of the crank pins Pc of theforged product 6 is deviated from specified positions. Specifically,among the crank pins Pc of the forged product 6, the first and thirdcrank pins P1 c and P3 c at opposite ends are eccentric in the directionperpendicular to the axial direction in the same direction. The secondcrank pins P2 c in the center is eccentric in the direction opposite toan eccentric direction of the first and third crank pins P1 c and P3 c.The arms Ac of the forged product 6 have an axial thickness equal tothat of arms A of the forged crankshaft having the final shape.

The blank for finish forging 5 has a shape that is generally inagreement with the shape of the forged product 6. Specifically, therough journal portions Jb of the blank for finish forging 5 have anaxial length equal to that of the journals J of the forged crankshafthaving the final shape (journals Jc of forged product 6). The roughcrank pin portions Pb of the blank for finish forging 5 have an axiallength equal to that of the crank pins P of the forged crankshaft havingthe final shape (crank pins Pc of forged product 6), and the amount ofeccentricity in the direction perpendicular to the axial direction isthe same between them. However, the placement angle of the blank forfinish forging 5 is, like the forged product 6, deviated from thespecified positions. The rough arm portions Ab of the blank for finishforging 5 have an axial thickness equal to that of the arms A of theforged crankshaft having the final shape (arms Ac of forged product 6).

In contrast, the rough journal portions Ja of the preform blank 4 havean axial length equal to that of the rough journal portions Jb of theblank for finish forging 5, i.e., that of the journals J of the forgedcrankshaft (journals Jc of forged product 6). The rough crank pinportions Pa of the preform blank 4 have an axial length equal to that ofthe rough crank pin portions Pb of the blank for finish forging 5, i.e.,that of the crank pins P of the forged crankshaft (crank pins Pc offorged product 6), but have a smaller amount of eccentricity than thatof the rough crank pin portions Pb of the blank for finish forging 5.Specifically, among the rough crank pin portions Pa of the preform blank4, the first and third rough crank pin portions P1 a and P3 a atopposite ends are eccentric in the same direction with an amount ofeccentricity thereof equal to about a half of an amount of eccentricityin the crank pins P of the forged crankshaft. On the other hand, thesecond rough crank pin portion P2 a in the center is eccentric in adirection opposite to an eccentric direction of the first and thirdrough crank pin portions P1 a and P3 a with an amount of eccentricityequal to about a half of an amount of eccentricity in the crank pin P ofthe forged crankshaft.

Among the rough arm portions Aa of the preform blank 4, the weightedrough arm portions Aa (the first, second, fifth and sixth rough armportions A1 a, A2 a, A5 a, and A6 a) have an axial thickness greaterthan that of the respective weighted rough arm portions Ab of the blankfor finish forging 5, i.e., weighted arms A of the forged crankshaft(the weighted arms Ac of the forged product 6). On the other hand, theoval rough arm portions Aa of the preform blank 4 (the third and fourthrough arm portions A3 a and A4 a) have an axial thickness equal to thatof the respective oval rough arm portions Ab of the blank for finishforging 5, i.e., the oval arms A of the forged crankshaft (the oval armAc).

2-2. Process for Manufacturing Forged Crankshaft

FIG. 9 is a schematic diagram illustrating a process for manufacturing aforged crankshaft according to the second embodiment. As shown in FIG.9, the process for manufacturing the forged crankshaft for thethree-cylinder engine of the second embodiment includes a firstpreforming step, a second preforming step, a finish forging step, and atwisting step, and also includes a trimming step before the twistingstep and a coining step after the twisting step as necessary.

The first preforming step is a step in which the preform blank 4described above is obtained. The second preforming step is a step inwhich the blank for finish forging 5 described above having the finalshape of the forged crankshaft excluding the placement angle of crankpins is obtained from the preform blank 4 described above by using aforming apparatus described in FIG. 10 below. The finish forging step isa step in which the blank for finish forging 5 is supplied to beprocessed by finish forging, whereby the forged product 6 describedabove having the final shape of the forged crankshaft excluding theplacement angle of crank pins is obtained.

The twisting step is a step in which the twisted product 7 describedabove is obtained. In the twisting step, in a state in which thejournals and the crank pins of the forged product 6 described above areheld and retained, the journals are twisted around these axial centersin order to adjust the placement angle of the crank pins of the forgedproduct 6 to the placement angle of the crank pins of the forgedcrankshaft. In this manner, the twisted product 7 having a final shapethat is in agreement with the shape of the crankshaft of the forgedcrankshaft including the placement angle can be obtained.

2-3. Apparatus for Forming Blank for Finish Forging

FIG. 10 is a longitudinal sectional view showing a configuration of theforming apparatus according to the second embodiment. FIG. 10illustrates, as an example, the forming apparatus configured to form theblank for finish forging 5 from the preform blank 4 shown in FIG. 8. Itshould be noted that in the longitudinal sectional view shown in FIG.10, all parts of the rough crank pin portions are actually on the sameplane.

In the forming apparatus of the second embodiment shown in FIG. 10, thepreform blank 4 is placed in the dies in a manner such that an eccentricdirection of the rough crank pin portions Pa is in the verticaldirection. For example, the preform blank 4 is placed in a manner suchthat the first and third rough crank pin portions P1 a and P3 a arepositioned in the upper side and the second rough crank pin portion P2 ais positioned in the lower side. The preform blank 4 placed in such amanner is formed into the blank for finish forging 5. Other than this,the same configuration is shared with the forming apparatus of the firstembodiment shown in FIG. 4, thus the detailed description thereof willbe omitted.

FIGS. 11A and 11B are longitudinal sectional views illustrating aprocess for forming the blank for finish forging using the formingapparatus according to the second embodiment shown in FIG. 10. Amongthese figures, FIG. 11A shows a forming state at an initial stage and11B shows a forming state at the completion.

As shown in FIG. 11A, the preform blank 4 is placed in the lower movablejournal dies 10B, stationary journal die 9B, movable crank pin dies 12,and auxiliary crank pin dies 13, and then the downward movement of thepress machine is performed. This allows the movable journal dies 10U and10B holding and retaining each rough journal portion Ja to move axiallytoward the reference crank pin die 11 in contact with the second roughcrank pin portion P2 a. Concurrently, the movable crank pin dies 12 andthe auxiliary crank pin dies 13 in contact with the first and thirdrough crank pin portions P1 a and P3 a are moved axially toward thereference crank pin die 11. By this operation, in the preform blank 4,the weighted rough arm portions Aa are axially compressed by the movablejournal dies 10U and 10B, the stationary journal dies 9U and 9B, thereference crank pin die 11, and the movable crank pin dies 12, while theaxial lengths of the rough journal portions Ja and the rough crank pinportions Pa are maintained. Then, the thickness of the weighted rougharm portions Aa is reduced to the thickness of the weighted rough armportions Ab of the blank for finish forging 5 (see FIG. 11B).

Also, in coordination with the axial movement of the movable journaldies 10U and 10B as well as that of the movable crank pin dies 12 andthe auxiliary crank pin dies 13, the reference crank pin dies 11 and themovable crank pin dies 12 press the rough crank pin portions Pa of thepreform blank 4 in the direction perpendicular to the axial direction bythe operation of each hydraulic cylinders 16. By this operation, therough crank pin portions Pa of the preform blank 4 are displaced in thedirection perpendicular to the axial direction, thus despite that theplacement angle of the rough crank pin portions Pa is deviated from thespecified positions, the amount of eccentricity thereof is increased tothe amount of eccentricity of the rough crank pin portions Pb of theblank for finish forging 5 (see FIGS. 8 and 11B).

In this manner, it is possible to form, from the preform blank 4 withouta flash, the blank for finish forging 5 without a flash, which has ashape generally in agreement with the shape of the forged crankshaft forthe three-cylinder engine (forged final product) excluding the placementangle of the crank pins P. By supplying such a blank for finish forging5 without a flash for finish forging, and performing finish forging withit, it is possible to obtain the forged product 6 having the final shapeof the forged crankshaft for the three-cylinder engine but excluding theplacement angle of the crank pins, although some minor amount of flashis generated. Then, by performing the twisting on the forged product 6,it is possible to obtain the final shape of the forged crankshaft forthe three-cylinder engine including the placement angle of the crankpins. Therefore, forged crankshafts for three-cylinder engines can bemanufactured with high material utilization and also with highdimensional accuracy regardless of their shapes.

3. Third Embodiment

A third embodiment is based on the configuration of the first and secondembodiments described above. The third embodiment includes modificationsin the relevant parts of the configuration, so that a final shape of aforged crankshaft can be formed as desired in finish forging stepwithout applying the twisting step in a process of manufacturing theforged crankshaft.

3-1. Preform Blank, Blank for Finish Forging, and Forged Product

FIG. 12 is a diagram schematically showing the shapes of a preform blankto be processed by the forming apparatus, a blank for finish forgingformed therefrom, and a forged product after finish forging, in themanufacturing method of the third embodiment. FIG. 12 shows, similar toFIGS. 2 and 8 above, how a three-cylinder four-counterweight crankshaftis manufactured.

As shown in FIG. 12, the preform blank 4 of the third embodiment has acrankshaft shape that is approximate to the shape of a forged crankshaft1 for the three-cylinder four-counterweight, but is generally in a roughshape. The preform blank 4 includes four rough journal portions Ja,three rough crank pin portions Pa, a rough front part portion Fra, arough flange portion Fla, and six rough arm portions Aa. The blank forfinish forging 5 of the third embodiment is formed from the preformblank 4 described above using a forming apparatus, details of which willbe provided below. The blank for finish forging 5 includes four roughjournal portions Jb, three rough crank pin portions Pb, a rough frontpart portion Frb, a rough flange portion Flb, and six rough arm portionsAb. The forged product 6 of the third embodiment is obtained from theblank for finish forging 5 described above by finish forging. The forgedproduct 6 includes four journals Jc, three crank pins Pc, a front partFrc, a flange Flc, and six arms Ac.

The forged product 6 has a shape that is in agreement with the shape ofthe crankshaft (forged final product) including a placement angle of thecrank pins Pc. Specifically, the journals Jc of the forged product 6have an axial length equal to that of the journals J of the forgedcrankshaft having the final shape. The crank pins Pc of the forgedproduct 6 have an axial length equal to that of the crank pins P of theforged crankshaft having the final shape. Further, the crank pins Pc ofthe forged product 6 have the same amount of eccentricity in a directionperpendicular to an axial direction and the same placement angle of 120°as the crank pins P of the forged crankshaft having the final shape,thus they are placed at the specified positions. The arms Ac of theforged product 6 have an axial thickness equal to that of arms A of theforged crankshaft having the final shape.

In contrast, the rough journal portions Jb of the blank for finishforging 5 have an axial length equal to that of the journals Jc offorged product 6, i.e., that of the journals J of the forged crankshaft.The rough crank pin portions Pb of the blank for finish forging 5 havean axial length equal to that of the crank pins Pc of forged product 6,i.e., that of the crank pins P of the forged crankshaft, but both theamount of eccentricity and the placement angle of the rough crank pinportions Pb are deviated from the specified positions. Specifically,among the rough crank pin portions Pb of the blank for finish forging 5,the first and third rough crank pin portions P1 b and P3 b at oppositeends are eccentric in the opposite direction to each other with anamount of eccentricity equal to a √3/2 of an amount of eccentricity inthe crank pins P of the forged crankshaft. On the other hand, the secondrough crank pin portion P2 b is not eccentric and has an amount ofeccentricity of zero. The rough arm portions Ab of the blank for finishforging 5 have an axial thickness equal to that of the arms A of theforged crankshaft having the final shape (arms Ac of forged product 6).

Also, the rough journal portions Ja of the preform blank 4 have an axiallength equal to that of the rough journal portions Jb of the blank forfinish forging 5, i.e., that of the journals J of the forged crankshaft(journals Jc of forged product 6). The rough crank pin portions Pa ofthe preform blank 4 have an axial length equal to that of the roughcrank pin portions Pb of the blank for finish forging 5, i.e., that ofthe crank pins P of the forged crankshaft (crank pins Pc of forgedproduct 6). However, among the rough crank pin portions Pa of thepreform blank 4, the first and third rough crank pin portions P1 a andP3 a have a smaller amount of eccentricity than that of the rough crankpin portions Pb of the blank for finish forging 5, and are eccentric inthe opposite direction to each other with an amount of eccentricity lessthan the √3/2 of the amount of eccentricity in the crank pins P of theforged crankshaft. On the other hand, the second rough crank pin portionP2 a has an amount of eccentricity of zero, similar to the second roughcrank pin portion P2 b in the blank for finish forging 5.

Among the rough arm portions Aa of the preform blank 4, the weightedrough arm portions Aa (the first, second, fifth and sixth rough armportions A1 a, A2 a, A5 a, and A6 a) have an axial thickness greaterthan that of the respective weighted rough arm portions Ab of the blankfor finish forging 5, i.e., weighted arms A of the forged crankshaft(the weighted arms Ac of the forged product 6). On the other hand, theoval rough arm portions Aa of the preform blank 4 (the third and fourthrough arm portions A3 a and A4 a) have an axial thickness equal to thatof the respective oval rough arm portions Ab of the blank for finishforging 5, i.e., the oval arms A of the forged crankshaft (the oval armAc).

3-2. Process for Manufacturing Forged Crankshaft

FIG. 13 is a schematic diagram illustrating a process for manufacturingthe forged crankshaft according to the third embodiment. As shown inFIG. 13, the process for manufacturing the forged crankshaft for thethree-cylinder engine of the third embodiment includes a firstpreforming step, a second preforming step, and a finish forging step,and also includes a trimming step and a coining step as necessary.

The first preforming step is a step in which the preform blank 4described above is obtained. The second preforming step is a step inwhich the blank for finish forging 5 described above having the finalshape of the forged crankshaft is obtained from the preform blank 4described above excluding the amount of eccentricity and the placementangle of all the crank pins, by using a forming apparatus described inFIG. 14 below.

The finish forging step is a step in which the forged product 6described above is obtained. In the finish forging step, the blank forfinish forging 5 described above is supplied to be processed by pressforging with a pair of upper and lower dies in a state in which thefirst and third rough crank pin portions are horizontally positioned,whereby all rough crank pin portions are pressed in the verticaldirection perpendicular to the axial direction. By this operation, theforged product 6 having a shape in agreement with the shape of thecrankshaft of the forged crankshaft having the final shape can beobtained including the placement angle of the crank pins.

3-3. Apparatus for Forming Blank for Finish Forging

FIG. 14 is a longitudinal sectional view showing a configuration of theforming apparatus according to the third embodiment. FIG. 14illustrates, as an example, the forming apparatus that is used informing the blank for finish forging 5 from the preform blank 4 shown inFIG. 12. It should be noted that in the longitudinal sectional viewshown in FIG. 14, all parts of the rough crank pin portions are actuallyon the same plane.

The forming apparatus of the third embodiment shown in FIG. 14 differsfrom the forming apparatus of the first embodiment shown in FIG. 4 andthe forming apparatus of the second embodiment shown in FIG. 10 largelyin the following. In the forming apparatus of the third embodiment, thepreform blank 4 is placed in the dies in a manner such that the firstand third rough crank pin portions P1 a and P3 a which are eccentric inthe opposite direction to each other are vertically positioned. Thepreform blank 4 in this manner is formed into the blank for finishforging 5. In this process, the reference crank pin die 11 disposed inthe location of the second rough crank pin portion P2 a is constrainedfrom moving not only in the axial direction but also in the directionperpendicular to the axial direction. For this reason, the referencecrank pin die 11 of the third embodiment is, unlike the one in the firstand second embodiments, not coupled to a hydraulic cylinder, instead,directly mounted to one of the upper die holder 23 and the lower dieholder 22. To the other one, the auxiliary crank pin die 13 forming apair with the reference crank pin die 11 is directly mounted. FIG. 14shows a mode in which the reference crank pin die 11 is mounted to theupper die holder 23 while the auxiliary crank pin die 13 is mounted tothe lower die holder 22.

Further, in the forming apparatus of the third embodiment, the movablecrank pin dies 12 and the auxiliary crank pin dies 13 are disposed atlocations of the rough crank pin portions P1 a and P3 a. However, avertical arrangement of the movable crank pin dies 12 and the auxiliarycrank pin dies 13 is reversed between the locations of the first andthird rough crank pin portions P1 a and P3 a. This is because the firstand third rough crank pin portions P1 a and P3 a are eccentric in theopposite direction to each other in the vertical direction. FIG. 14shows a mode in which the auxiliary crank pin die 13 at the location ofthe first rough crank pin portion P1 a and the movable crank pin die 12at the location of the third rough crank pin portion P3 a are disposedon the upper side while the movable crank pin die 12 at the location ofthe first rough crank pin portion P1 a and the auxiliary crank pin die13 at the location of the third rough crank pin portion P3 a aredisposed on the lower side.

FIGS. 15A and 15B are longitudinal sectional views illustrating aprocess for forming a blank for finish forging using the formingapparatus according to the third embodiment shown in FIG. 14. Amongthese figures, FIG. 15A shows a forming state at an initial stage and15B shows a forming state at the completion.

As shown in FIG. 15A, the preform blank 4 is placed in the lower movablejournal dies 10B, stationary journal die 9B, movable crank pin dies 12,and auxiliary crank pin dies 13, and then the downward movement of thepress machine is performed. Then, the movable journal dies 10U and 10Band the stationary journal dies 9U and 9B are caused to hold and retainthe rough journal portions Ja of the preform blank 4 therebetween fromabove and below, and concurrently, the reference crank pin die 11 andthe auxiliary crank pin dies 13 are caused to hold and retain the secondrough crank pin portion P2 a therebetween from above and below, bringinginto a state in which the first and third rough crank pin portions P1 aand P3 a are in contact with the movable crank pin dies 12. In thisstate, the lowering of the press machine is continued. This allows themovable journal dies 10U and 10B holding and retaining each roughjournal portion Ja to move axially toward the reference crank pin die 11holding and retaining the second rough crank pin portion P2 a.Concurrently, the movable crank pin dies 12 and the auxiliary crank pindies 13 in contact with the first and third rough crank pin portions P1a and P3 a are moved axially toward the reference crank pin die 11. Bythis operation, in the preform blank 4, the weighted rough arm portionsAa are axially compressed by the movable journal dies 10U and 10B, thestationary journal dies 9U and 9B, the reference crank pin die 11, andthe movable crank pin dies 12, while the axial lengths of the roughjournal portions Ja and the rough crank pin portions Pa are maintained.Then, the thickness of the weighted rough arm portions Aa is reduced tothe thickness of the weighted rough arm portions Ab of the blank forfinish forging 5 (see FIG. 15B).

Also, in coordination with the axial movement of the movable journaldies 10U and 10B as well as that of the movable crank pin dies 12 andthe auxiliary crank pin dies 13, the movable crank pin dies 12 press thefirst and third rough crank pin portions P1 a and P3 a of the preformblank 4 in the vertical direction perpendicular to the axial directionby the operation of each hydraulic cylinders 16. By this operation, thefirst and third rough crank pin portions P1 a and P3 a of the preformblank 4 are displaced in the vertical direction perpendicular to theaxial direction, thus the amount of eccentricity thereof in the oppositedirection to each other is increased to an amount of eccentricity equalto a √3/2 of an amount of eccentricity in the crank pins P of the forgedcrankshaft (see FIGS. 12 and 15B). On the other hand, the location ofthe second rough crank pin portion P2 a of the preform blank 4 in thevertical direction perpendicular to the axial direction remainsunchanged before and after the forming, thus the amount of eccentricitythereof remains zero.

In this manner, it is possible to form, from the preform blank 4 withouta flash, the blank for finish forging 5 without a flash, which has ashape generally in agreement with the shape of the forged crankshaft forthe three-cylinder engine (forged final product) excluding the amount ofeccentricity and the placement angle of all the crank pins P. Such ablank for finish forging 5 without a flash is supplied for finishforging, and finish forging is performed with it in a state in which thefirst and third rough crank pin portions P1 a and P3 a are horizontallypositioned. In this process, by pressing all the rough crank pinportions of the blank for finish forging 5 in the vertical directionperpendicular to the axial direction so as to displace them to thespecified positions, it is possible to obtain the final shape of theforged crankshaft for the three-cylinder engine including the contourshape of arms, and the amount of eccentricity and the placement angle ofthe crank pins, although some minor amount of flash is generated.Therefore, forged crankshafts for three-cylinder engines can bemanufactured with high material utilization and also with highdimensional accuracy regardless of their shapes.

4. Fourth Embodiment

A fourth embodiment includes modifications of the configuration of thethird embodiment.

4-1. Preform Blank, Blank for Finish Forging, and Forged Product

FIG. 16 is a diagram schematically showing the shapes of a preform blankto be processed by the forming apparatus, a blank for finish forgingformed therefrom, and a forged product after finish forging, in themanufacturing method of the fourth embodiment.

As shown in FIG. 16, the forged product 6 of the fourth embodimentmaintains the same shape as the forged product 6 of the third embodimentshown in FIG. 12.

In contrast, the blank for finish forging 5 of the fourth embodimentdiffers from the blank for finish forging 5 of the third embodimentshown in FIG. 12 in the following. As shown in FIG. 16, central secondrough crank pin portion P2 b of the blank for finish forging 5 of thefourth embodiment is configured to be eccentric in a directionperpendicular to an eccentric direction of the first and third roughcrank pin portions P1 b and P3 b at opposite ends with an amount ofeccentricity equal to that of the crank pin Pc of the forged product 6,i.e., that of the crank pin P of the forged crankshaft.

Further, the preform blank 4 of the fourth embodiment differs from thepreform blank 4 of the third embodiment shown in FIG. 12 in thefollowing. As shown in FIG. 16, central second rough crank pin portionP2 a of the preform blank 4 of the fourth embodiment is configured to beeccentric in a direction perpendicular to an eccentric direction offirst and third rough crank pin portions P1 a and P3 a at opposite endswith an amount of eccentricity equal to that of the crank pin P of theforged crankshaft (crank pin Pc of forged product 6), the configurationsimilar to that of the blank for finish forging 5.

4-2. Process for Manufacturing Forged Crankshaft

FIG. 17 is a schematic diagram illustrating a process for manufacturinga forged crankshaft according to the fourth embodiment. As shown in FIG.17, the process for manufacturing the forged crankshaft of the fourthembodiment, similar to the third embodiment shown in FIG. 13, includes afirst preforming step, a second preforming step, and a the finishforging step, and also includes a trimming step and a coining step asnecessary.

The first preforming step is a step in which the preform blank 4described above is obtained.

The second preforming step is a step in which the blank for finishforging 5 described above is obtained. In the second preforming step,the same forming apparatus used in the third embodiment shown in FIGS.14, 15A and 15B is used. It should be noted that in the longitudinalsectional view shown in FIG. 14, the second rough crank pin portion inthe fourth embodiment is in reality located either in the front or backside of the paper.

In the second preforming step of the fourth embodiment, as similarlyfound in the third embodiment shown in FIGS. 14, 15A, and 15B, thepreform blank 4 is placed in the lower journal die 10B, the stationaryjournal dies 9B, the movable crank pin die 12, and the auxiliary crankpin dies 13, and then the downward movement of the press machine isperformed. This allows the movable journal dies 10U and 10B holding andretaining each rough journal portion Ja, and the movable crank pin dies12 and the auxiliary crank pin dies 13 in contact with the first andthird rough crank pin portions P1 a and P3 a to move axially toward thereference crank pin die 11 holding and retaining the second rough crankpin portion P2 a. In conjunction with this movement, in the preformblank 4, the weighted rough arm portions Aa are axially compressed whilethe axial lengths of the rough journal portions Ja and the rough crankpin portions Pa are maintained. Then, the thickness of the weightedrough arm portions Aa is reduced to the thickness of the weighted rougharm portions Ab of the blank for finish forging 5.

Further, the first and third rough crank pin portions P1 a and P3 a arepressed by the movable crank pin dies 12 in the vertical directionperpendicular to the axial direction. In this manner, the first andthird rough crank pin portions P1 a and P3 a of the preform blank 4become eccentric in the opposite direction to each other with an amountof eccentricity increased to a √3/2 of an amount of eccentricity in thecrank pins P of the forged crankshaft. On the other hand, the locationof the second rough crank pin portion P2 a of the preform blank 4 in thedirection perpendicular to the axial direction remains unchanged beforeand after the forming, thus an amount of eccentricity remains the sameas that of the crank pin P of the forged crankshaft.

In this manner, it is possible to form, from the preform blank 4 withouta flash, the blank for finish forging 5 without a flash, which has ashape generally in agreement with the shape of the forged crankshaft forthe three-cylinder engine (forged final product) excluding the amount ofeccentricity and the placement angle of the first and third crank pinsP1 and P3.

The finish forging step is a step in which the forged product 6described above is obtained. In the finish forging step, the blank forfinish forging 5 is supplied to be processed for finish forging in astate that in which the first and third rough crank pin portions arehorizontally positioned. In this process, by pressing the first andthird rough crank pin portions P1 b and P3 b of the blank for finishforging 5 in the vertical direction perpendicular to the axial directionso as to displace them to the specified positions, it is possible toobtain the forged product 6 having a shape in agreement with the shapeof the crankshaft of the forged crankshaft for the three-cylinder enginehaving the final shape including the contour shape of arms, and theamount of eccentricity and the placement angle of the crank pins,although some minor amount of flash is generated.

The present invention is not limited to the embodiments described above,and various modifications may be made without departing from the spiritand scope of the present invention. For example, the mechanism forcausing the movable journal dies to move axially is not limited to theone described in the above embodiments, in which a wedge mechanism of apress machine is employed. Alternatively, a link mechanism may beemployed, or a hydraulic cylinder or a servo motor or the like may beemployed in place of the press machine. Furthermore, the mechanism forcausing the crank pin dies to move in the direction perpendicular to theaxial direction is not limited to a hydraulic cylinder, and it may be aservo motor.

In the embodiments described above, the inclined surfaces are providedto the end surface of the movable journal dies, as wedge mechanisms tomove axially the movable journal dies, and wedges are providedcorrespondingly to these inclined surfaces. As wedge mechanisms insteadto this, a block having an inclined surface may be fixed to a sidesection outside the first impression and the second impression of themovable journal die, and a wedge may be provided correspondingly to theinclined surface of this block.

Furthermore, the embodiments described above have such a configurationthat the upper die holder is secured to the upper pressure pad while thelower die holder is resiliently supported on the lower pressure pad onwhich the wedges are installed, and the upper and lower movable journaldies are allowed to move by the wedges, but alternatively, the functionsof the upper section and the lower section may be reversed. Theconfiguration may also be such that the upper and lower die holders areresiliently supported on the corresponding pressure pads, and thatwedges are installed on both pressure pads so that the upper and lowermovable journal dies are caused to move by their corresponding wedges.

Furthermore, in the above embodiments, the auxiliary crank pin dies aremovable only axially, but they may be made to be movable also in adirection toward the crank pin dies forming pairs. In this case, thecrank pin dies and the auxiliary crank pin dies hold and retain therough crank pin portions Pa therebetween from above and below andmeanwhile move in the direction perpendicular to the axial directioncooperatively with each other.

Furthermore, the embodiments described above have such a configurationthat the rough crank pin portions Pa are pressed in the verticaldirection by moving the crank pin dies in the direction perpendicular tothe axial direction, however the configuration may also be such that thelocations of the crank pin dies and the journal dies are changed so asto horizontally press the rough crank pin portions Pa.

INDUSTRIAL APPLICABILITY

The present invention is useful in manufacturing forged crankshafts forthree-cylinder engines.

REFERENCE SIGNS LIST

-   1: forged crankshaft-   J, J1 to J4: journals-   P, P1 to P3: crank pins-   Fr: front part-   Fl: flange-   A, A1 to A6: crank arms-   2: billet-   4: preform blank-   Ja, J1 a to J4 a: rough journal portions-   Pa, P1 a to P3 a: rough crank pin portions-   Fra: rough front part portion-   Fla: rough flange portion-   Aa, A1 a to A6 a: rough crank arm portions-   5: blank for finish forging-   Jb, J1 b to J4 b: rough journal portions of blank for finish forging-   Pb, P1 b to P3 b: rough crank pin portions of blank for finish    forging-   Frb: rough front part portion of blank for finish forging-   Flb: rough flange portion of blank for finish forging-   Ab, A1 b to A6 b: rough crank arm portions of blank for finish    forging-   5 a: fin flaws-   6: forged product-   Jc, J1 c to J4 c: journals of forged product-   Pc, P1 c to P3 c: crank pins of forged product-   Frc: front part of forged product-   Flc: flange of forged product-   Ac, A1 c to A6 c: crank arms of forged product-   7: twisted product-   Jd, J1 d to J4 d: journals of twisted product-   Pd, P1 d to P3 d: crank pins of twisted product-   Frd: front part of twisted product-   Fld: flange of twisted product-   Ad, A1 d to A6 d: crank arms of twisted product-   9U, 9B: stationary journal dies-   9Ua, 9Ba: first impression of stationary journal die-   9Ub, 9Bb: second impression of stationary journal die-   9Uc, 9Bc: third impression of stationary journal die-   10U, 10B: movable journal dies-   10Ua, 10Ba: first impression of movable journal die-   10Ub, 10Bb: second impression of movable journal die-   11: reference crank pin die-   11 a: impression-   12: movable crank pin die-   12 a: impression-   13: auxiliary crank pin die-   13 a: impression-   14U, 14B: inclined surfaces of journal dies for first and fourth    rough journal portions-   16: hydraulic cylinder-   20: lower pressure pad-   21: upper pressure pad-   22: lower die holder-   23: upper die holder-   24: resilient member-   25: support post-   26: wedge

The invention claimed is:
 1. An apparatus for forming a blank for finishforging for a forged crankshaft for a three-cylinder engine in whichthird and fourth crank arms connecting with a second crank pin in acenter have no balance weights and remaining crank arms have balanceweights, the apparatus configured to form, from a preform blank, in aprocess of manufacturing the forged crankshaft for a three-cylinderengine, the blank for finish forging to be subjected to finish forgingby which a final shape of the forged crankshaft is obtained, the preformblank including: rough journal portions having an axial length equal toan axial length of journals of the forged crankshaft; rough crank pinportions having an axial length equal to an axial length of crank pinsof the forged crankshaft; third and fourth rough crank arm portionscorresponding to the third and fourth crank arms of the forgedcrankshaft, having an axial thickness equal to an axial thickness ofsuch crank arms; and weighted rough crank arm portions corresponding toweighted crank arms having the balance weights of the forged crankshaft,having an axial thickness greater than an axial thickness of such crankarms, the rough crank pin portions of the preform blank having a smalleramount of eccentricity in a direction perpendicular to an axialdirection than an amount of eccentricity of the crank pins of the forgedcrankshaft, the forming apparatus comprising: a reference crank pin diedisposed at a location of the second rough crank pin portion, thereference crank pin die configured to be brought into contact with thesecond rough crank pin portion, the reference crank pin die configuredto move in a direction perpendicular to the axial direction, but beconstrained from moving axially, while being in contact with sidesurfaces of the third and fourth rough crank arm portions through whichthe third and fourth rough crank arm portions connect with the secondrough crank pin portion; movable crank pin dies disposed at locations ofthe corresponding first and third rough crank pin portions at oppositeends, the movable crank pin dies configured to be brought into contactwith the first and third rough crank pin portions, the movable crank pindies configured to move axially toward the reference crank pin die andin the direction perpendicular to the axial direction, while being incontact with side surfaces of the rough crank arm portions through whichthe rough crank arm portions connect with the first and third roughcrank pin portions; stationary journal dies disposed at locations of therough journal portions connecting with the third and fourth rough crankarm portions, the stationary journal dies configured to hold and retainsuch rough journal portions therebetween in the direction perpendicularto the axial direction, the stationary journal dies configured to beconstrained from moving axially while being in contact with sidesurfaces of the third and fourth rough crank arm portions; and movablejournal dies disposed at locations of the corresponding rough journalportion excluding the rough journal portions connecting with the thirdand fourth rough crank arm portions, the movable journal dies configuredto hold and retain such rough journal portions therebetween in thedirection perpendicular to the axial direction, the movable journal diesconfigured to move axially toward the reference crank pin die whilebeing in contact with side surfaces of the rough crank arm portionsthrough which the rough crank arm portions connect with such roughjournal portions, wherein, in a state where the rough journal portionsare held and retained by the stationary journal dies and the movablejournal dies, and the rough crank pin portions are contacted by thereference crank pin die and the movable crank pin dies, the movablejournal dies are moved axially, the movable crank pin dies are movedaxially and in the direction perpendicular to the axial direction, andthe reference crank pin die is moved in the direction perpendicular tothe axial direction, thereby compressing the weighted rough crank armportions in the axial direction so as to reduce the thickness thereof tothe thickness of the weighted crank arms of the forged crankshaft, andpressing the rough crank pin portions in the direction perpendicular tothe axial direction so as to increase the amount of eccentricity thereofto the amount of eccentricity of the crank pins of the forgedcrankshaft.
 2. The apparatus for forming a blank for finish forging fora forged crankshaft for a three-cylinder engine according to claim 1,wherein the reference crank pin die and the movable crank pin dies eachincludes an auxiliary crank pin die disposed at a location outside ofthe corresponding rough crank pin portion, opposite to the side wherethe reference crank pin die and the movable crank pin dies arecontacted, and wherein in conjunction with the axial movement of themovable journal dies as well as that of the movable crank pin dies andthe auxiliary crank pin dies forming pairs therewith, a movement of themovable crank pin dies in the direction perpendicular to the axialdirection is controlled in a manner that the rough crank pin portions tobe deformed by pressing reach the auxiliary crank pin dies after spacesbetween the movable journal dies, and the movable crank pin dies and theauxiliary crank pin dies are filled.
 3. The apparatus for forming ablank for finish forging for a forged crankshaft for a three-cylinderengine according to claim 2, wherein, provided that a total length ofmovement of the movable crank pin dies in the direction perpendicular tothe axial direction is a 100% length of movement thereof, when the axialmovement of the movable journal dies that are adjacent to such movablecrank pin dies is completed, a length of movement of such movable crankpin dies in the direction perpendicular to the axial direction is 90% orless of the total length of movement, and thereafter, the movement ofsuch movable crank pin dies in the direction perpendicular to the axialdirection is completed.
 4. The apparatus for forming a blank for finishforging for a forged crankshaft for a three-cylinder engine according toclaim 1, wherein the reference crank pin die, the movable crank pindies, the stationary journal dies, and the movable journal dies aremounted on a press machine that is capable of being moved downward alongthe direction perpendicular to the axial direction, and wherein, by thedownward movement of the press machine, the stationary journal dies andthe movable journal dies are caused to hold and retain the rough journalportions therebetween, the reference crank pin die and the movable crankpin dies are brought into contact with the rough crank pin portions, andwith continued downward movement of the press machine, the movablejournal dies are moved axially by wedge mechanisms, and the movablecrank pin dies are caused to move axially by the movement of the movablejournal dies.
 5. The apparatus for forming a blank for finish forgingfor a forged crankshaft for a three-cylinder engine according to claim4, wherein the wedge mechanisms have different wedge angles for each ofthe movable journal dies.
 6. The apparatus for forming a blank forfinish forging for a forged crankshaft for a three-cylinder engineaccording to claim 4 or 5, wherein the reference crank pin die and themovable crank pin dies are coupled to hydraulic cylinders and caused tomove in the direction perpendicular to the axial direction by drivingthe hydraulic cylinders.
 7. An apparatus for forming a blank for finishforging for a forged crankshaft for a three-cylinder engine in whichthird and fourth crank arms connecting with a second crank pin in acenter have no balance weights and remaining crank arms have balanceweights, the apparatus configured to form, from a preform blank, in aprocess of manufacturing the forged crankshaft for a three-cylinderengine, the blank for finish forging to be subjected to finish forgingby which a final shape of the forged crankshaft is obtained, the preformblank including: rough journal portions having an axial length equal toan axial length of journals of the forged crankshaft; first, second andthird rough crank pin portions having an axial length equal to an axiallength of crank pins of the forged crankshaft; third and fourth roughcrank arm portions corresponding to the third and fourth crank arms ofthe forged crankshaft, having an axial thickness equal to an axialthickness of such crank arms; and weighted rough crank arm portionscorresponding to weighted crank arms having the balance weights of theforged crankshaft, having an axial thickness greater than an axialthickness of such crank arms, the forming apparatus comprising:reference crank pin die disposed at a location of the second rough crankpin portion, the reference crank pin die configured to be brought intocontact with the second rough crank pin portion, the reference crank pindie configured to be constrained from moving axially while being incontact with side surfaces of the third and fourth rough crank armportions through which the rough crank arm portions connect with thesecond rough crank pin portion; movable crank pin dies disposed atlocations of the corresponding first and third rough crank pin portions,the movable crank pin dies configured to be brought into contact withthe first and third rough crank pin portions, the movable crank pin diesconfigured to move axially toward the reference crank pin die and in thedirection perpendicular to the axial direction, while being in contactwith side surfaces of the rough crank arm portions through which therough crank arm portions connect with the first and third rough crankpin portions; stationary journal dies disposed at locations of the roughjournal portions connecting with the third and fourth rough crank armportions, the stationary journal dies configured to hold and retain suchrough journal portions therebetween in the direction perpendicular tothe axial direction, the stationary journal dies configured to beconstrained from moving axially, while being in contact with sidesurfaces of the third and fourth rough crank arm portions; and movablejournal dies disposed at locations of the corresponding rough journalportion excluding the rough journal portions connecting with the thirdand fourth rough crank arm portions, the movable journal dies configuredto hold and retain such rough journal portions therebetween in thedirection perpendicular to the axial direction, the movable journal diesconfigured to move axially toward the reference crank pin die whilebeing in contact with side surfaces of the rough crank arm portionsthrough which the rough crank arm portions connect with such roughjournal portions, wherein, in a state where the rough journal portionsare held and retained by the stationary journal dies and the movablejournal dies, and the rough crank pin portions are contacted by thereference crank pin die and the movable crank pin dies, the movablejournal dies are moved axially and the movable crank pin dies are movedaxially and in the direction perpendicular to the axial direction,thereby compressing the weighted rough crank arm portions in the axialdirection so as to reduce the thickness thereof to the thickness of theweighted crank arms of the forged crankshaft, and pressing the first andthird rough crank pin portions in the direction perpendicular to theaxial direction but in an opposite direction to each other, so as toincrease an amount of eccentricity thereof to the √3/2 of an amount ofeccentricity of the crank pins of the forged crankshaft.
 8. Theapparatus for forming a blank for finish forging for a forged crankshaftfor a three-cylinder engine according to claim 7, wherein the apparatusconfigured to form the blank for finish forging from a preform blank,the preform blank including: rough journal portions having an axiallength equal to an axial length of journals of the forged crankshaft;first, second and third rough crank pin portions having an axial lengthequal to an axial length of crank pins of the forged crankshaft; thirdand fourth rough crank arm portions corresponding to the third andfourth crank arms of the forged crankshaft, having an axial thicknessequal to an axial thickness of such crank arms; and weighted rough crankarm portions corresponding to weighted crank arms having the balanceweights of the forged crankshaft, having an axial thickness greater thanan axial thickness of such crank arms, the first and third rough crankpin portions at opposite ends of the preform blank having an amount ofeccentricity in a direction perpendicular to the axial direction in theopposite direction to each other, the amount of eccentricity thereofbeing smaller than a √3/2 of an amount of eccentricity of the crank pinsof the forged crankshaft, the second rough crank pin portion in thecenter of the preform blank having an amount of eccentricity of zero inthe direction perpendicular to the axial direction or having an amountof eccentricity in a direction perpendicular to an eccentric directionof the first and third rough crank pin portions, the amount ofeccentricity thereof being equal to the amount of eccentricity of thecrank pins of the forged crankshaft.
 9. The apparatus for forming ablank for finish forging for a forged crankshaft for a three-cylinderengine according to claim 7, wherein the reference crank pin die and themovable crank pin dies each includes an auxiliary crank pin die disposedat a location outside of the corresponding rough crank pin portion,opposite to the side where the reference crank pin die and the movablecrank pin dies are contacted, and wherein in conjunction with the axialmovement of the movable journal dies as well as that of the movablecrank pin dies and the auxiliary crank pin dies forming pairs therewith,a movement of the movable crank pin dies in the direction perpendicularto the axial direction is controlled in a manner that the rough crankpin portions to be deformed by pressing reach the auxiliary crank pindies after spaces between the movable journal dies, and the movablecrank pin dies and the auxiliary crank pin dies are filled.
 10. Theapparatus for forming a blank for finish forging for a forged crankshaftfor a three-cylinder engine according to claim 9, wherein, provided thata total length of movement of the movable crank pin dies in thedirection perpendicular to the axial direction is a 100% length ofmovement thereof, when the axial movement of the movable journal diesthat are adjacent to such movable crank pin dies is completed, a lengthof movement of such movable crank pin dies in the directionperpendicular to the axial direction is 90% or less of the total lengthof movement, and thereafter, the movement of such movable crank pin diesin the direction perpendicular to the axial direction is completed. 11.The apparatus for forming a blank for finish forging for a forgedcrankshaft for a three-cylinder engine according to claim 7, wherein thereference crank pin die, the movable crank pin dies, the stationaryjournal dies, and the movable journal dies are mounted on a pressmachine that is capable of being moved downward along the directionperpendicular to the axial direction, and wherein, by the downwardmovement of the press machine, the stationary journal dies and themovable journal dies are caused to hold and retain the rough journalportions therebetween, the reference crank pin die and the movable crankpin dies are brought into contact with the rough crank pin portions, andwith continued downward movement of the press machine, the movablejournal dies are moved axially by wedge mechanisms, and the movablecrank pin dies are caused to move axially by the movement of the movablejournal dies.
 12. The apparatus for forming a blank for finish forgingfor a forged crankshaft for a three-cylinder engine according to claim11, wherein the wedge mechanisms have different wedge angles for each ofthe movable journal dies.
 13. The apparatus for forming a blank forfinish forging for a forged crankshaft for a three-cylinder engineaccording to claim 11, wherein the movable crank pin dies are coupled tohydraulic cylinders and caused to move in the direction perpendicular tothe axial direction by driving the hydraulic cylinders.
 14. A method forforming a forged crankshaft for a three-cylinder engine in which thirdand fourth crank arms connecting with a second crank pin in a centerhave no balance weights and remaining crank arms have balance weights,comprising the successive steps of: a first preforming step for forming,as the preform blank to be supplied to the forming apparatus accordingto claim 1, a preform blank in which first and third rough crank pinportions at opposite ends have an amount of eccentricity in a directionperpendicular to the axial direction in the opposite direction to eachother, the amount of eccentricity thereof being equal to a √3/2 of anamount of eccentricity of the crank pins of the forged crankshaft, and asecond rough crank pin portion in the center has an amount ofeccentricity in the direction perpendicular to the axial direction, in adirection perpendicular to an eccentric direction of the first and thirdrough crank pin portions, the amount of eccentricity thereof beingsmaller than the amount of eccentricity of the crank pins of the forgedcrankshaft; a second preforming step for forming, as the blank forfinish forging, a blank for finish forging in which a final shape of theforged crankshaft is formed including a placement angle of the crankpins using the forming apparatus; and a finish forging step for, byperforming finish forging on the blank for finish forging, forming aforged product having the final shape of the forged crankshaft includingthe placement angle of the crank pins.
 15. A method for forming a forgedcrankshaft for a three-cylinder engine in which third and fourth crankarms connecting with a second crank pin in a center have no balanceweights and remaining crank arms have balance weights, comprising thesuccessive steps of: a first preforming step for forming, as the preformblank to be supplied to the forming apparatus according to claim 1, apreform blank in which first and third rough crank pin portions atopposite ends have an amount of eccentricity in a directionperpendicular to the axial direction in the same direction, the amountof eccentricity thereof being smaller than an amount of eccentricity ofthe crank pins of the forged crankshaft, and a second rough crank pinportion in the center has an amount of eccentricity in the directionperpendicular to the axial direction, in the opposite direction of aneccentric direction of the first and third rough crank pin portions, theamount of eccentricity thereof being smaller than the amount ofeccentricity of the crank pins of the forged crankshaft; a secondpreforming step for forming, as the blank for finish forging, a blankfor finish forging in which a final shape of the forged crankshaft isformed excluding a placement angle of the crank pins using the formingapparatus; a finish forging step for forming, by performing finishforging on the blank for finish forging, a forged product having thefinal shape of the forged crankshaft excluding the placement angle ofthe crank pins; and a twisting step for adjusting the placement angle ofthe crank pins of the forged product to the placement angle of the crankpins of the forged crankshaft.
 16. A method for forming a forgedcrankshaft for a three-cylinder engine in which third and fourth crankarms connecting with a second crank pin in a center have no balanceweights and remaining crank arms have balance weights, comprising thesuccessive steps of: a first preforming step for forming, as the preformblank to be supplied to the forming apparatus according to claim 7, apreform blank in which the first and third rough crank pin portions atopposite ends have an amount of eccentricity in a directionperpendicular to the axial direction in the opposite direction to eachother, the amount of eccentricity thereof being smaller than a √3/2 ofan amount of eccentricity of the crank pins of the forged crankshaft,and the second rough crank pin portion in the center has the amount ofeccentricity of zero in the direction perpendicular to the axialdirection; a second preforming step for forming, using the formingapparatus, as the blank for finish forging, a blank for finish forgingin which first and third rough crank pin portions at opposite ends havean amount of eccentricity in the direction perpendicular to the axialdirection in the opposite direction to each other, the amount ofeccentricity thereof being equal to the √3/2 of the amount ofeccentricity of the crank pins of the forged crankshaft, and a secondrough crank pin portion in the center remains the same amount ofeccentricity in the direction perpendicular to the axial direction asthe preform blank; and a finish forging step for forming a forgedproduct having a final shape of the forged crankshaft including aplacement angle of the crank pins by performing finish forging on theblank for finish forging in a state in which the first and third roughcrank pin portions at opposite ends are horizontally positioned wherebyall the rough crank pin portions are pressed in the directionperpendicular to the axial direction.
 17. A method for forming a forgedcrankshaft for a three-cylinder engine in which third and fourth crankarms connecting with a second crank pin in a center have no balanceweights and remaining crank arms have balance weights, comprising thesuccessive steps of: a first preforming step for forming, as the preformblank to be supplied to the forming apparatus according to claim 7, apreform blank in which first and third rough crank pin portions atopposite ends have an amount of eccentricity in a directionperpendicular to the axial direction in the opposite direction to eachother, the amount of eccentricity thereof being smaller than a √3/2 ofan amount of eccentricity of the crank pins of the forged crankshaft,and a second rough crank pin portion in the center has an amount ofeccentricity in the direction perpendicular to the axial direction, in adirection perpendicular to an eccentric direction of the first and thirdrough crank pin portions, the amount of eccentricity thereof being equalto the amount of eccentricity of the crank pins of the forgedcrankshaft; a second preforming step for forming, using the formingapparatus, as the blank for finish forging, a blank for finish forgingin which the first and third rough crank pin portions at opposite endshave an amount of eccentricity in the direction perpendicular to theaxial direction in the opposite direction to each other, the amount ofeccentricity thereof being equal to the √3/2 of the amount ofeccentricity of the crank pins of the forged crankshaft, and the secondrough crank pin portion in the center remains the same amount ofeccentricity in the direction perpendicular to the axial direction asthe preform blank; and a finish forging step for forming a forgedproduct having a final shape of the forged crankshaft including aplacement angle of the crank pins by performing finish forging on theblank for finish forging in a state in which the first and third roughcrank pin portions at opposite ends are horizontally positioned wherebythe first and third rough crank pin portions are pressed in thedirection perpendicular to the axial direction.